[source-postgres] support cdc against a read-replica (continuation)

[theyueli]

What

Fixes #45396

In a previous community PR , we added support to query LSN against a read replica. However, this is still not sufficient as we also need to retrieve current transaction id to create Debezium state together with the LSN.

However, we currently retrieve transaction id using select * from txid_current() which can only be performed against a primary. We will approximate this in this patch if we read against a replica:

SELECT 
    CASE 
        WHEN pg_is_in_recovery() THEN txid_snapshot_xmin(txid_current_snapshot())
        ELSE txid_current()
    END AS current_txid;

Here, txid_snapshot_xmin(txid_current_snapshot()) provides a lower bound to txid_current(), and can be executed on the replica.

How

Review guide

User Impact

Can this PR be safely reverted and rolled back?

• [X ] YES 💚
• NO ❌

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[theyueli]

@assaadhjb we need at least another change I think.

[theyueli]

/bump-version type=patch changelog="Support CDC against a read-replica (continuation)"

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✅ Changes applied successfully. (34fa335)