Improve discovery process for user without permissions to avoid fetching not permitted schemas and tables

[sashaNeshcheret]

Current Behavior

In db2, cockroachdb and postgres source connectors during discovery we are fetching tables and schemas to which user does not have any permissions.

Expected Behavior

In that case user should not see any tables

Logs

If applicable, please upload the logs from the failing operation.
For sync jobs, you can download the full logs from the UI by going to the sync attempt page and
clicking the download logs button at the top right of the logs display window.

Steps to Reproduce

1. Use db2, cockroachdb or postgres source
2. Create user without any select permissions
3. Run discovery with the user.
4. All tables are shown.

Are you willing to submit a PR?

Remove this with your answer.