move sign pkg to pkg/sign

[sallyom]

Summary

https://github.com/sigstore/cosign/issues/1660

Currently the signing logic lives in cmd/cosign/cli/sign, this PR moves signing logic to pkg/sign. Moving to pkg/sign allows other projects to include in their tools.

Release Note

NONE

Documentation

NONE

[haydentherapper]

Before moving over all of these functions into pkg, we will need to review what's public as not all functions should be publicly available, add documentation, etc.

[codecov-commenter]

Codecov Report

Merging #2074 (873e8c1) into main (eaa3a8a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2074   +/-   ##
=======================================
  Coverage   26.33%   26.33%           
=======================================
  Files         129      129           
  Lines        7564     7564           
=======================================
  Hits         1992     1992           
  Misses       5317     5317           
  Partials      255      255           

Impacted Files	Coverage Δ
cmd/cosign/cli/policy_init.go	1.38% <ø> (ø)
cmd/cosign/cli/sign.go	0.00% <ø> (ø)
cmd/cosign/cli/signblob.go	0.00% <ø> (ø)
cmd/cosign/cli/verify/verify.go	0.00% <ø> (ø)
pkg/sign/sign.go	14.69% <ø> (ø)
pkg/sign/sign_blob.go	0.00% <ø> (ø)

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[github-actions[bot]]

This PR was closed because it has been stalled for 10 days with no activity.