Add edit mirror API endpoint

[lecafard]

Fixes #1534

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Also, to speed up things, if you could kindly "Allow edits and access to secrets by maintainers" in the
PR settings, as this allows us to rebase the PR on master, fix conflicts, run coverage and help with
implementing code and tests.

Description of the Change

The current API does not have an option to edit like the CLI does. Adding this for parity.

The endpoint is POST /api/mirrors/{name} and returns 201 if successful.

There's currently little unit testing around the API as there is currently not too much mocking infrastructure so just resorted to writing system tests.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated (i assume this is autogenerated from the go comments)
• author name in AUTHORS

[codecov]

Codecov Report

❌ Patch coverage is 74.02597% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.64%. Comparing base (a65f79e) to head (50b9663).

Files with missing lines	Patch %	Lines
api/mirror.go	73.68%	14 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1535      +/-   ##
==========================================
- Coverage   76.96%   76.64%   -0.32%     
==========================================
  Files         160      160              
  Lines       14754    14831      +77     
==========================================
+ Hits        11355    11367      +12     
- Misses       2266     2330      +64     
- Partials     1133     1134       +1     

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