Add attest-blob command

[priyawadhwa]

Built on top of #1265, thank you mlieberman85!

This is just the signing piece, we will need cosign verify-attest-blob as well which I'll do in a follow-up PR.
Once that's in I'll add in the unit and integration tests!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 92 lines in your changes missing coverage. Please review.

Project coverage is 29.84%. Comparing base (d795dcb) to head (5b76402).
Report is 1045 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cosign/cli/attest_blob.go	0.00%	54 Missing ⚠️
cmd/cosign/cli/options/attest_blob.go	0.00%	37 Missing ⚠️
cmd/cosign/cli/commands.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2286      +/-   ##
==========================================
- Coverage   30.18%   29.84%   -0.34%     
==========================================
  Files         131      133       +2     
  Lines        8089     8181      +92     
==========================================
  Hits         2442     2442              
- Misses       5323     5415      +92     
  Partials      324      324              

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[vaikas]

Thanks so much for doing this! Would it be possible to add an e2e test for this functionality like here (hopefully mainly copy&paste if we want a new test leg, or just add a new test case in the existing one (sign/verify) (not worrying about the now overbaked testfile name :) ):
https://github.com/sigstore/cosign/blob/main/.github/workflows/kind-verify-attestation.yaml#L91

I'd be happy to add this myself but might not get to it for day or twoish

[priyawadhwa]

@vaikas would it be ok if i did it in a follow-up pr? my next one is gonna be for the verify-blob-attestation command so i think it'll be easier to include the test once we have both signing & verification!

[vaikas]

Oh, absolutely, that's perfetto!