Add API to fetch git notes

[nitul1991]

This PR is the first in a series of commits related to exposing api endpoints to interact with git notes.

This first PR, simply creates an endpoint to fetch a git note corresponding to a commit.

close #15373

[codecov-commenter]

Codecov Report

Merging #16649 (2e6fbae) into main (23d438f) will decrease coverage by 0.02%.
The diff coverage is 67.56%.

@@            Coverage Diff             @@
##             main   #16649      +/-   ##
==========================================
- Coverage   45.40%   45.37%   -0.03%     
==========================================
  Files         756      758       +2     
  Lines       85279    85230      -49     
==========================================
- Hits        38718    38674      -44     
+ Misses      40300    40292       -8     
- Partials     6261     6264       +3     

Impacted Files	Coverage Δ
modules/git/notes_nogogit.go	71.42% <60.00%> (-3.58%)	⬇️
routers/api/v1/repo/notes.go	69.23% <69.23%> (ø)
routers/api/v1/api.go	78.47% <100.00%> (+0.02%)	⬆️
modules/notification/mail/mail.go	35.29% <0.00%> (-2.95%)	⬇️
services/pull/check.go	26.20% <0.00%> (-2.07%)	⬇️
routers/api/v1/repo/commits.go	54.87% <0.00%> (-1.60%)	⬇️
modules/notification/ui/ui.go	60.86% <0.00%> (-1.45%)	⬇️
models/commit_status.go	60.41% <0.00%> (-1.33%)	⬇️
routers/web/repo/commit.go	27.62% <0.00%> (-1.33%)	⬇️
models/issue_comment.go	51.40% <0.00%> (-1.18%)	⬇️
... and 23 more

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[nitul1991]

I've incorporated all the review comments and squashed all the commits into one meaningful commit.

[6543]

🚀

[6543]

lgtm