enhance: add link, host, and custom labeling

[ecrupper]

Adding the following labels:

• "io.vela.build.host" — this will be the worker that built the image
• "io.vela.build.link" — this will be the exact link to the Vela build

Also added VELA_BUILD_CUSTOM_LABELS as an input for folks to write their own metadata. Part of that development included a safety check to make sure users aren't overwriting any values (hence the use of a label map, which in turn made the testing function a little tricky but it works with sorting)

[codecov]

Codecov Report

Attention: Patch coverage is 43.03797% with 45 lines in your changes missing coverage. Please review.

Project coverage is 39.63%. Comparing base (0e69356) to head (6fe5cf4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/vela-kaniko/main.go	0.00%	35 Missing ⚠️
cmd/vela-kaniko/repo.go	72.22%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   32.76%   39.63%   +6.87%     
==========================================
  Files           8        8              
  Lines         586      830     +244     
==========================================
+ Hits          192      329     +137     
- Misses        393      498     +105     
- Partials        1        3       +2     

Files with missing lines	Coverage Δ
cmd/vela-kaniko/plugin.go	94.52% <100.00%> (+1.57%)	⬆️
cmd/vela-kaniko/repo.go	90.00% <72.22%> (-10.00%)	⬇️
cmd/vela-kaniko/main.go	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[KellyMerrick]

lgtm