Feature request or enhancement

For feature requests or enhancements, provide this information, please:

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.

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()`.
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