feat(api): add exclusive GET attrs for /projects/:id/members

[dedlocc]

#2964 introduced support for passing a username to /groups/:id/members. The same should also be possible for /projects/:id/members.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.55%. Comparing base (d6b1b0a) to head (2455f87).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2969      +/-   ##
==========================================
+ Coverage   92.40%   96.55%   +4.15%     
==========================================
  Files          95       95              
  Lines        6040     6040              
==========================================
+ Hits         5581     5832     +251     
+ Misses        459      208     -251     

Flag	Coverage Δ
api_func_v4	82.68% <ø> (?)
cli_func_v4	83.22% <ø> (+0.04%)	⬆️
unit	88.52% <ø> (ø)

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Files with missing lines	Coverage Δ
gitlab/v4/objects/members.py	94.82% <ø> (+1.72%)	⬆️

... and 36 files with indirect coverage changes