Get primary key for tables in snapshot from replica identity [DBZ-2037]

Migrated from [DBZ-2037](https://issues.redhat.com/browse/DBZ-2037)

Currently, we have several tables in our DB that we'd like to replicate, and that don't have a primary key. Now, since they're in a DB with logical replication turned on (else Debezium would be kind of pointless), we've manually altered the {{REPLICA IDENTITY}} of those tables to be {{FULL}}. We then have to duplicate this logic in the Debezium configuration so as not to send null keys.

It would be a nice bonus if Debezium could utilize the same logic as Postgres for unconfigured tables - or if not the exact same logic, at least taking a hint from the replica identity if the table has no primary key or special configuration in config.

Start by reading the migrated DBZ-2037 issue and reviewing PostgreSQL replica identity behavior, especially FULL for tables without primary keys. Trace how Debezium chooses keys for unconfigured tables; done means its key selection follows the replica identity without requiring duplicated configuration.

Written by the indexing model from the issue text.