Logical replication of tables without primary keys automatically falls back to FULL IDENTITY.

• Introduce a new GUC: logical_replication_fallback_to_full_identity.
• When enabled, if a table being logically replicated has no primary key,
  the system automatically uses REPLICATION IDENTITY FULL for that table.
• This setting can be applied at the database level, so large systems do
  not need to enable it cluster-wide unless desired.
• When the WAL sender transmits relation metadata, if fallback has
  occurred, it explicitly reports FULL as the replication identity to the
  subscriber, so there is limited impact on the subscriber.

This PR implements the new feature. Close #1226