sql: implement pg_my_temp_schema and pg_is_other_temp_schema

[cockroach-teamcity]

Exalate commented:

cockroachdb/cockroach#69909 --- Release note (sql change): The pg_my_temp_schema builtin function now properly returns the OID of the active session's temporary schema, if one exists. Release justification: None, waiting for v22.1.

Jira Issue: DOC-1722

[github-actions]

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