"source" block for Postgres events shouldn't contain "last_snapshot_record" field [DBZ-797]

Migrated from [DBZ-797](https://issues.redhat.com/browse/DBZ-797)

This is only needed within the KC offset to detect an on-going snapshot after a restart, but it's not needed within "source" in the actual message payload. Ideally, when changing this, the PG connector should make use of the same structure as the Oracle one, which clearly separates between {{SourceInfo}} (representing the "source" element of messages) and {{OffsetContext}} (managing the KC offset and links to {{SourceInfo}}).

Start by comparing the PostgreSQL connector's source payload and KC offset handling with the Oracle connector's SourceInfo and OffsetContext separation. Trace where last_snapshot_record is added to the source element and confirm it is needed only in the KC offset for restart detection. Done means PostgreSQL messages no longer include that field while snapshot restart detection remains intact.

Written by the indexing model from the issue text.