feat(api): add support for gitlab service account

[SachinKSingh28]

Adds support for creating group service account: https://docs.gitlab.com/ee/api/groups.html#service-accounts

https://github.com/python-gitlab/python-gitlab/issues/2812

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.51%. Comparing base (7ec3189) to head (5256a9c). Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2851      +/-   ##
==========================================
- Coverage   96.52%   96.51%   -0.02%     
==========================================
  Files          90       91       +1     
  Lines        5872     5907      +35     
==========================================
+ Hits         5668     5701      +33     
- Misses        204      206       +2     

Flag	Coverage Δ
api_func_v4	82.39% <100.00%> (+0.15%)	:arrow_up:
cli_func_v4	83.61% <100.00%> (+0.02%)	:arrow_up:
unit	88.33% <100.00%> (+0.03%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
gitlab/v4/objects/__init__.py	100.00% <100.00%> (ø)
gitlab/v4/objects/groups.py	95.90% <100.00%> (+0.04%)	:arrow_up:
gitlab/v4/objects/service_accounts.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[SachinKSingh28]

@nejch addressed your comments! Ready for a review again.

[nejch]

Thanks again @SachinKSingh28, I applied a nit left over from the first round myself so we don't block this anymore. I'll squash and merge :+1: