[GSoC Milestone 1] Migrate resource calls from RESTEasy to GitLab4J-API

[harsh-ps-2003]

This PR migrates the GitLab Plugin's resource calls from RESTEasy to GitLab4J-API without affecting the older Webhook implementation. The Proxy Configuration is currently non-functional.

An attempt has been made to adapt Docker Based Functional Tests accordingly.

[krisstern]

@basil I have re-enabled the tests and everything seems to be in working order for this PR

[krisstern]

Review Checklist

• [ ] Make sure unit tests are passing
• [ ] Make sure integration tests are passing
• [x] Review logic of code by module
• [x] Review logic of mapping between RESTEasy implementation and gitlab4j-api implementation
• [x] Review quality of PR

To be signed off by @krisstern.

[krisstern]

@harsh-ps-2003 I have made some requests for changes. I will need to test interactively once again after these changes have been made, and can merge the PR after that.

[krisstern]

Thanks for your hard work @harsh-ps-2003! I tried it interactively with a push event and it worked. For testing for merge request I am seeing the below:

Which may mean that I need a bit working on my own setup.

So to follow up, once I have set up a proper merge request for the merge test the test was passed: