PostgreSQL Compatibility: `SET TIME ZONE '<offset>'` fails with invalid value parameter error

[funtuan]

Describe the bug
When using the PostgreSQL-compatible syntax to set the session timezone with an offset string (e.g., '00:00:00'), doltgresql throws an invalid value error. This syntax works correctly in standard PostgreSQL.

To Reproduce
Steps to reproduce the behavior:

1. Connect to the doltgresql database using psql or any other client.
2. Run the following SQL command:

   SET TIME ZONE '00:00:00';

Expected behavior
The command should execute successfully and set the session's time zone to the specified offset, matching the behavior of standard PostgreSQL.

Actual behavior
The command fails and returns the following error:

ERROR: ERROR:  invalid value for parameter "TimeZone": "00:00:00" (errno 1105) (sqlstate HY000)

Environment details:

• doltgresql version / image tag: dolthub/doltgresql:0.55.6
• OS: macOS