[Feature] In a Postgres datasource, allow choosing a specific schema, and/or only display accessible tables

## Summary

In a Postgres datasource, Instead of automatically querying pg_catalog for all tables/schemas, allow for a specific "schema" to be chosen, with the default being "public".

## Motivation

Currently, when choosing a Postgres database with many schemas besides "public", *all* the tables from *all* schemas are displayed in left panel, whether the database user has access to them or not. (This is a quirk of Postgres, that querying the pg_catalog table shows all schemas and tables whether privileges have been granted or not.)

## Additional Context

#There are two approaches, I've seen to this problem, ideally both may be implemented.
1. add a Schema field to the form where datasource is setup. Default is "public".
2. Metabase handles this beautifully, when adding a datasource, they query the privileges to only display tables that the database user has access to. It would be great if Appsmith could do this as well.
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Attached is a screenshot of the current result of adding a database with many schemas.