[PATCH] Add permit_unlogged_tables GUC to control unlogged table creation.

Hello,
This patch implements a new GUC parameter "permit_unlogged_tables" which
allows admins to disable creation of unlogged tables or altering the
existing tables to be unlogged.

This is useful in environments with strict compliance requirements as well
as multi-tenant environments where the provider needs to guarantee
durability of customer data.

The default is "true", which is to say that behavior would remain unchanged.

Implementation
- Boolean GUC with PGC_SIGHUP context (reloadable without restart)
- Check in "DefineRelation()" to block "CREATE UNLOGGED TABLE" commands.
- Check in "ATPrepCmd()" to block "ALTER TABLE SET UNLOGGED" commands.
- "ALTER TABLE SET LOGGED" commands remain unaffected and should always
work.

Attached the patch for review. Happy to make adjustments based on feedback.

Thanks !

Best,
Harinath