Add `_ab_cdc_txid` to CDC output

[gpind]

What area the feature impact?

Connectors

Revelant Information

For audit logs, it's essential to associate transaction metadata with CDC events. For example, an application might write user IDs or request IDs to a transaction_metadata table every time it opens a transaction, with the transaction's ID as primary key. Then if we had a field like _ab_cdc_txid we could tie that metadata to any CDC event.

I don't know which sources have a concept of a "transaction ID", but Postgres does at a minimum (Debezium exposes it as txId; see here).

[gpind]

Thoughts? I can start on a PR if people agree it's a good idea, though I could also use a hint about where to start.

[gpind]

Ping. This blocks us from using Airbyte for CDC.