Support builder signing

[stormqueen1990]

Changes

• Implement builder signing with cosign.

This is still a work in progress.

Fixes #942

[codecov-commenter]

Codecov Report

Merging #954 (017a84c) into main (b7b98e2) will decrease coverage by 0.26%. The diff coverage is 54.05%.

@@            Coverage Diff             @@
##             main     #954      +/-   ##
==========================================
- Coverage   69.87%   69.61%   -0.27%     
==========================================
  Files         118      119       +1     
  Lines        5600     5683      +83     
==========================================
+ Hits         3913     3956      +43     
- Misses       1306     1340      +34     
- Partials      381      387       +6     

Impacted Files	Coverage Δ
pkg/apis/build/v1alpha2/builder_lifecycle.go	0.00% <0.00%> (ø)
pkg/apis/build/v1alpha2/builder_types.go	25.00% <ø> (ø)
pkg/cnb/create_builder.go	72.41% <42.85%> (-4.06%)	:arrow_down:
pkg/cosign/image_signer.go	72.81% <50.00%> (-17.02%)	:arrow_down:
pkg/reconciler/builder/builder.go	46.75% <66.66%> (+2.63%)	:arrow_up:
pkg/reconciler/clusterbuilder/clusterbuilder.go	46.75% <66.66%> (+2.63%)	:arrow_up:
pkg/apis/build/v1alpha2/build_pod.go	98.28% <100.00%> (ø)
pkg/apis/build/v1alpha1/build_validation.go	78.12% <0.00%> (-8.24%)	:arrow_down:
pkg/apis/build/v1alpha2/build_priority.go	0.00% <0.00%> (ø)

[stormqueen1990]

Closing this in favour of #1208.