Summary:
- This diff adds schema details to the query diagnostics bundle, within the file named `schema_details.txt`.
- We output only the schema of the tables mentioned within the bundled query.
- We reuse the queries within DescribeOneTableDetails() function of describe.c to extract schema information.
- For executing SQL queries within postgres backend we use  SPI framework.
- Executing SQL queries requires us to have a database connection, thus the background worker is registered with `BGWORKER_BACKEND_DATABASE_CONNECTION` bgw_flag and start_time is changed to `BgWorkerStart_RecoveryFinished`.

Sample output of schema_details.txt:

```
"Table Name: test_table"

- Table information:
|Table Name        |Table Groupname |Colocated |
+------------------+----------------+----------+
|public.test_table |                |false     |

- Columns:
|Column      |Type                     |Collation |Nullable |Default           |Storage  |Stats Target |Description |
+------------+-------------------------+----------+---------+------------------+---------+-------------+------------+
|id          |integer                  |          |not null |                  |plain    |             |            |
|name        |character varying(100)   |          |not null |                  |extended |             |            |
|description |text                     |          |         |                  |extended |             |            |
|updated_at  |timestamp with time zone |          |         |CURRENT_TIMESTAMP |plain    |             |            |
|is_active   |boolean                  |          |         |true              |plain    |             |            |
|price       |numeric(10,2)            |          |         |                  |main     |             |            |

- Indexes:
|Name                          |Index Definition                                                                                          |Constraint Definition    |
+------------------------------+----------------------------------------------------------------------------------------------------------+-------------------------+
|test_table_pkey               |CREATE UNIQUE INDEX test_table_pkey ON ONLY test_table USING lsm (id HASH)                                |PRIMARY KEY (id)         |
|test_table_id_description_key |CREATE UNIQUE INDEX test_table_id_description_key ON ONLY test_table USING lsm (id HASH, description ASC) |UNIQUE (id, description) |
|idx_test_table_active         |CREATE INDEX idx_test_table_active ON ONLY test_table USING lsm (is_active HASH) WHERE is_active = true   |                         |
|idx_test_table_name           |CREATE INDEX idx_test_table_name ON ONLY test_table USING lsm (name HASH)                                 |                         |

- Check constraints:
|Name                 |Constraint Definition       |
+---------------------+----------------------------+
|check_price_positive |CHECK (price >= 0::numeric) |

- Policies:
|Name        |Type       |Applicable Roles |USING Expression   |With CHECK Expression |Applicable Command |
+------------+-----------+-----------------+-------------------+----------------------+-------------------+
|test_policy |Permissive |ALL ROLES        |(is_active = true) |                      |SELECT             |

- Statistics:
|OID   |TABLE      |Namespace |Statistics Name |Columns     |N-Dinstinct Enabled |Functional Dependencies Enabled |
+------+-----------+----------+----------------+------------+--------------------+--------------------------------+
|16403 |test_table |public    |test_statistics |name, price |No                  |Yes                             |

- Rules:
|Name      |Rule Definition                                                                                                                         |
+----------+----------------------------------------------------------------------------------------------------------------------------------------+
|test_rule |CREATE RULE test_rule AS     ON DELETE TO test_table DO INSTEAD  UPDATE test_table SET is_active = false   WHERE test_table.id = old.id |

============================================================

"Table Name: foreign_test_table"

- Table information:
|Table Name                |Table Groupname |Colocated |
+--------------------------+----------------+----------+
|public.foreign_test_table |                |false     |

- Columns:
|Column |Type                   |Collation |Nullable |Default |Storage  |Stats Target |Description |
+-------+-----------------------+----------+---------+--------+---------+-------------+------------+
|id     |integer                |          |         |        |plain    |             |            |
|name   |character varying(100) |          |         |        |extended |             |            |

- Foreign table information:
|Server Name    |FDW Options                                          |
+---------------+-----------------------------------------------------+
|foreign_server |schema_name 'public', table_name 'remote_test_table' |

============================================================
```

- As highlighted in worker_spi.c, it is important to be in a transactional state before calling `SPI_execute()` or `get_rel_name` from a background worker. Without this we hit an assertion failure `Assert(IsTransactionState());` within SearchCatCacheInternal() in catcache.c
- Further `pg_plan_query` expects to have an `ActiveSnapshotSet()` thus we `PushActiveSnapshot()` before calling `SPI_execute()`.
- Also, we haven't included queries to gather data on `inherited tables` as they are not yet supported by YB.
Jira: DB-11074

Test Plan: ./yb_build.sh --java-test TestYbQueryDiagnostics#checkSchemaDetails

Reviewers: asaha, telgersma

Reviewed By: telgersma

Subscribers: telgersma, yql

Differential Revision: https://phorge.dev.yugabyte.com/D36469