feat: add GitLab OAuth client

[almereyda]

This adds a GitLab OAuth client, based on the Google and GitHub examples.

This adds a new authentication_error_code() convenience method to oauth.py, in order to reduce code duplication.

To raise exceptions when OAuth is malconfigured, it also adds an additional OAUTH_NOT_CONFIGURED error.

This also adds execute permission to the files apiserver/bin/docker-entrypoint-beat.sh and apiserver/bin/docker-entrypoint-migrator.sh, as it was a dependency to being able to develop. (#4630)

There is a bit of code duplication between admin, web and spaces, but that may be a design choice.

This has been tested successfully against a custom GitLab instance and should work similarly against the default instance.

addresses #408
addresses #413
closes #4630

[almereyda]

Please review what when wrong with clicking on the "Resolve conflicts" button:

• https://github.com/makeplane/plane/pull/4828/conflicts

The introduction of the convenience method authentication_error_code() brings distinct error codes for each different case of self.provider and for the unknown case, which is rarely reached.

I'm unable to give a reproducer at the moment, but there was a case during development, when OAUTH_NOT_CONFIGURED was needed to be thrown.

[almereyda]

These changes basically help to resolve the code duplication discussed in #4828 (comment)

[almereyda]

Again, please excuse the earlier confusion with #4692 (comment) by incurring more work.

[almereyda]

Ready for review.

[sriramveeraghanta]

We have already squashed all your changes. We will review and fix things as a part of the internal release testing process. So closing this PR for now.