Summary:
Add GUC_EXPLAIN flag to YB-specific GUC parameters affecting query planning, so modified values are shown in EXPLAIN output with the SETTINGS option.

Method of Identification:
1. Extract the GUC parameters from `guc.c` where:
  * Name starts with `yb_`
  * Doesn't have `GUC_EXPLAIN`
2. From the extracted list, remove the ones that is not referenced in any of those files:
  * src/postgres/src/include/optimizer/*.h
  * src/postgres/src/backend/optimizer/*/*.c
  * src/postgres/src/backend/access/yb_access/yb_scan.c
  * src/postgres/src/backend/access/yb_access/yb_lsm.c
  * src/postgres/src/backend/commands/explain.c
3. Eye-ball the list and exclude the ones that won't affect planning.

The finalists for `GUC_EXPLAIN`:
```
yb_bnl_batch_size
yb_bnl_optimize_first_batch
yb_bypass_cond_recheck
yb_enable_base_scans_cost_model
yb_enable_batchednl
yb_enable_bitmapscan
yb_enable_distinct_pushdown
yb_enable_geolocation_costing
yb_enable_optimizer_statistics
yb_enable_parallel_append
yb_fetch_row_limit
yb_fetch_size_limit
yb_network_fetch_cost
yb_parallel_range_rows
yb_parallel_range_size
yb_prefer_bnl
yb_pushdown_is_not_null (hidden)
yb_pushdown_strict_inequality (hidden)
```

Used in the files listed above but excluded because they do not impact the plan:
```
yb_enable_memory_tracking
yb_enable_planner_trace
yb_explain_hide_non_deterministic_fields
yb_explicit_row_locking_batch_size
yb_hinted_uids
yb_lock_pk_single_rpc
```
Jira: DB-16279

Test Plan: ./yb_build.sh release --java-test 'org.yb.pgsql.TestPgRegressPlanner'

Reviewers: mihnea, gkukreja, william.mckenna

Reviewed By: gkukreja

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D45159