Add method to update code_owner_approval_required flag to Protected Branches API

[MikeWillCook]

Adds basic support for updating the code_owner_approval_required flag on an existing protected branch. See the GitLab API docs for intended usage.

Two notes for feedback:

1. The HTTP "PATCH" method isn't supported in the current client so this uses the X-HTTP-Method-Override: PATCH header as a workaround. This wires it thru AbstractApi to GitLabApiClient where it's turned into a POST with the override header. Anyone recommend another way?
2. Removed the integration test as it's a Premium or higher feature so can't be tested on the Community Edition image. (EDIT: Added this test back with handling for current version.)

Fixes #869

[jabby]

Hello thanks for your PR.

I did not check if it works, but I think you can use invocation(url, queryParams).method("PATCH", empty);

Can you try with my proposal ?

[MikeWillCook]

Thanks for the suggestion, @jabby. In fact I tried that first (after not finding a native patch() method) and found it doesn't work with the current client, and instead returns the following exception:

org.gitlab4j.api.GitLabApiException: java.net.ProtocolException: Invalid HTTP method: PATCH

There is some discussion of other workarounds for the underlying bug, but this approach seemed sufficient and less problematic to me. I'm open to pushback on that.

Also, to help in the future and at least exercise the method, I've added a test handling the current tested version with comments on what should happen with Premium.

[MikeWillCook]

Any other questions on this, @jabby ?