[12.1] Add missing award emoji methods to MergeRequests

[antonkomarev]

Adds the missing award_emoji write/read endpoints to
Gitlab\Api\MergeRequests — both for the merge request itself
and for its notes:

Method	Verb	Path
addAwardEmoji()	POST	.../merge_requests/:iid/award_emoji
showNoteAwardEmoji()	GET	.../merge_requests/:iid/notes/:note_id/award_emoji
addNoteAwardEmoji()	POST	.../merge_requests/:iid/notes/:note_id/award_emoji
removeNoteAwardEmoji()	DELETE	.../merge_requests/:iid/notes/:note_id/award_emoji/:award_id

Existing awardEmoji() (GET list) and removeAwardEmoji() (DELETE)
already covered the read/remove side for the MR itself, but there
was no way to actually add an emoji — and nothing at all for notes.

Naming follows the existing conventions on this class:

• show* for single-resource reads (showNote(), showDiscussion())
• add* / remove* for writes (addNote(), removeNote())

GitLab uses a flat /notes/:note_id/award_emoji path, so the note
methods also cover reactions on discussion thread comments —
callers just pass the note_id of the discussion note.

Reference: https://docs.gitlab.com/api/emoji_reactions/

Resurrects #794 (closed as stale after a request for test coverage);
this PR adds the missing tests and removeNoteAwardEmoji() for
symmetry.

[GrahamCampbell]

Thanks for getting started here. I've made some corrections, and moved it to #881.