feat(api): add support for the Draft notes API

[nejch]

Changes

See https://docs.gitlab.com/ee/api/draft_notes.html

Not returning anything in the publish methods as we only get a 204 from gitlab, see below:
https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/draft_notes.rb#L215-261

Documentation and testing

Please consider whether this PR needs documentation and tests. This is not required, but highly appreciated:

• Documentation in the matching docs section
• Unit tests and/or functional tests

[codecov]

Codecov Report

Merging #2728 (a4b28b0) into main (428b4fd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2728      +/-   ##
==========================================
+ Coverage   96.45%   96.47%   +0.01%     
==========================================
  Files          88       89       +1     
  Lines        5787     5812      +25     
==========================================
+ Hits         5582     5607      +25     
  Misses        205      205              

Flag	Coverage Δ
api_func_v4	82.45% <76.00%> (-0.03%)	⬇️
cli_func_v4	83.49% <76.00%> (-0.04%)	⬇️
unit	88.19% <100.00%> (+0.05%)	⬆️

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Files	Coverage Δ
gitlab/client.py	98.58% <100.00%> (+<0.01%)	⬆️
gitlab/v4/objects/__init__.py	100.00% <100.00%> (ø)
gitlab/v4/objects/draft_notes.py	100.00% <100.00%> (ø)
gitlab/v4/objects/merge_requests.py	89.04% <100.00%> (+0.15%)	⬆️