credentials: generalize prompt behavior

[pmrowla]

closes #225

[codecov-commenter]

Codecov Report

Patch coverage: 63.63% and project coverage change: -0.07 ⚠️

Comparison is base (b97d042) 81.25% compared to head (0d2d54d) 81.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #226      +/-   ##
==========================================
- Coverage   81.25%   81.19%   -0.07%     
==========================================
  Files          27       27              
  Lines        3814     3940     +126     
  Branches      668      692      +24     
==========================================
+ Hits         3099     3199     +100     
- Misses        608      623      +15     
- Partials      107      118      +11     

Impacted Files	Coverage Δ
src/scmrepo/git/credentials.py	57.68% <53.08%> (+5.48%)	⬆️
tests/test_credentials.py	100.00% <100.00%> (ø)

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[pmrowla]

GIT_ASKPASS integration looks like this in vscode:

2023-03-31.17-43-07.mp4

Example uses dvc get in the vscode terminal, but it would be the same if you ran an experiment that had an import in the pipeline

(the initial window fadeout is vscode opening a separate window asking if I want to login via the github vscode extension, if user logs in through the github extension it would skip prompting for username/password)