Changes MergeRequestApi:

- added methods "getDiffs()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/diffs

- add methods "getDiffVersions()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions

- add method "getDiffVersion()" and "getOptionalDiffVersion()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id

- deprecate "getMergeRequestDiffs()" and "getMergeRequestDiff()"

Fixes #993

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Fixes #931

Co-authored-by: Erkan <erkanerkisi>

Create `AcceptMergeRequestParams` class and method in `MergeRequestApi` to accept a merge request with several parameters. (#517)

Add additional parameters to `getMergeRequest()` for optional parameters from GitLab endpoint. (#511)

* Add Discussions model and API calls.

* Load forks of a project with Pager
* Load comments of a commit with Pager
* Load merge request changes
* Load merge request participants.
* Method to request repository contributors
* getMergeRequestChanges method needs list of diffs
* Fixed getFile method does not handle spaces in path correctly

* Added support to list MergeRequests by state
* Added getMergeRequests by State with paging support