Validate Mirror: PostgreSQL Checks

[Amogh-Bharadwaj]

This PR adds a Validate mirror button and corresponding logic to check if the postgres source peer and selected tables are ready to be mirrored. Moves validate peer logic to separate file

This PR covers:

• Check if replication permissions are there for user
• Check if we can do SELECT * FROM TABLE LIMIT 0. for all selected tables.
• If initial copy is selected, they can’t enter the replication slot.
• Check if publication they provide, is attached to all selected tables.
• If no tables show up, remind them check GRANT permissions.
• Validate peer: Don’t let you create < 12 version
• Link to docs in CREATE PEER page.