Remove Postgres adapter from Parse Server

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Parse Server should not contain database adapters that may not be used by the developer.

The Postgres adapter should be moved into its own repository which can be maintained separately and added by developers as needed. Given that MongoDB is likely the most widely used database adapter and it is also the best supported one in terms of features, it will be the default DB adapter and likely stay within the Parse Server repository to have a default adapter to run the tests. If technically possible and advantageous (testing, etc), also the MongoDB should be moved into its own repository over time.

Feature / Enhancement Description

In order to do that, the current Parse Server tests need to run using the external database adapter. We currently do not have a distinct set of DB tests, so all current tests need to be run and pass to know that the DB adapter works. If possible we could separate the tests and run only the specific DB test in the external adapter CI.

Changed needed for external DB adapters:

• Add test exclusion by ID feature https://github.com/parse-community/parse-server/issues/8714
• Create repo that pulls Parse Server tests and runs the full test suite but using the external DB adapter https://github.com/parse-community/parse-server/issues/8788
• Move Postgres adapter from Parse Server to external repo

Start by locating the Postgres adapter in Parse Server and reviewing how the current full test suite exercises database adapters. Read issue #8788 for the planned external test repository and determine the adapter’s extraction boundaries. Done means the Postgres adapter is maintained externally and the relevant Parse Server and external-adapter tests pass.