PostgreSQL ReadOnly Incremental Snapshots [DBZ-5046]

Migrated from [DBZ-5046](https://issues.redhat.com/browse/DBZ-5046)

h1. Feature request or enhancement

For feature requests or enhancements, provide this information, please:
h2. Which use case/requirement will be addressed by the proposed feature?

This proposal would remove the requirement for a signaling table in Postgres to trigger Incremental Snapshots. I also wonder it this might allow Incremental Snapshots to be the default snapshot mode for the initial snapshot.
h2. Implementation ideas (optional)

Background:
* Postgres has a `pg_logical_emit_message()` command which allows arbitrary messages to be passed through the logical decoding chain. 
* These logical decoding messages have always been supported by `wal2json` and are supported by `pgoutput` since Postgres 14, however, before Postgres 14 `pgoutput` would not support this signaling mechanism.
* https://www.postgresql.org/docs/current/functions-admin.html

Implementation:
* Replace the high and low watermarks in signaling table with messages emitted using `pg_logical_emit_message()`.
* Replace `execute-snapshot` message in signaling table with message to `pg_logical_emit_message()`.
*  

The issue names no repository files or tests. Start with PostgreSQL documentation for pg_logical_emit_message(), then inspect the existing incremental snapshot signaling implementation. Done means evaluating replacement of signaling-table watermarks and execute-snapshot messages, including pgoutput support before PostgreSQL 14 and whether this enables the proposed default snapshot mode.

Written by the indexing model from the issue text.