sql: support PG_DIAG_* error diagnostic fields

[rafiss]

These fields are documented here: https://www.postgresql.org/docs/devel/libpq-exec.html

For example:

PG_DIAG_SCHEMA_NAME
    If the error was associated with a specific database object, the name of the schema containing that object, if any.

PG_DIAG_TABLE_NAME
    If the error was associated with a specific table, the name of the table. (Refer to the schema name field for the name of the table's schema.)

PG_DIAG_COLUMN_NAME
    If the error was associated with a specific table column, the name of the column. (Refer to the schema and table name fields to identify the table.)

Jira issue: CRDB-5209

[github-actions]

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