fix: #19020 Check permissions using native Postgres functions to support GROUPs and ROLEs based access control

[edeesis]

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What's changed and what's your intention?

Update the SQL queries being run in the PostgresValidator to properly take into account GROUP and ROLE permissions.

This PR updates both the RDS Role check (for servers running in AWS) and the SELECT privilege check, to confirm that the PG user being used has those roles

Before, you had to GRANT the role to the user itself and explicitly GRANT SELECTs if the user wasn't the owner of the table.

See details in #19020

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The Postgres CDC source now fully supports GROUPs and ROLEs instead of requiring explicit permissions on USERs.

[edeesis]

It's unclear why the Connector Node Integration Tests are failing. The one that's failing is

Error: Errors:
Error: PostgresSourceTest.testLines:145 » Execution io.grpc.StatusRuntimeException: UNKNOWN

I can look deeper, but if anyone has an idea, I'm happy for the help.

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