Add attest blob functionality

[mlieberman85]

This mostly copies the functionality of image attestation and blob signing.

Signed-off-by: Michael [email protected]

Summary

This will allow users to attest local blobs similar to attesting images, following a similar pattern to signing blobs/images. Currently the code allows for attesting local blobs only. It probably wouldn't take much effort to include just a hash to attest as not all blobs will be available locally for checksumming. This also supports printing attestation locally or uploading it to Rekor.

[mlieberman85]

This is still very much a WIP. Need to add some abstractions to deduplicate some of the code between attest and attestblob.

[mlieberman85]

I have some additional stuff I've changed/added. I'm also going to include signing of the hash directly.

[codecov-commenter]

Codecov Report

Merging #1265 (83034d3) into main (1258512) will decrease coverage by 0.34%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1265      +/-   ##
==========================================
- Coverage   33.44%   33.10%   -0.35%     
==========================================
  Files         146      148       +2     
  Lines        9340     9428      +88     
==========================================
- Hits         3124     3121       -3     
- Misses       5843     5933      +90     
- Partials      373      374       +1     

Impacted Files	Coverage Δ
cmd/cosign/cli/attestblob.go	0.00% <0.00%> (ø)
cmd/cosign/cli/commands.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/attestblob.go	0.00% <0.00%> (ø)
pkg/cosign/tuf/client.go	61.68% <0.00%> (-0.82%)	:arrow_down:

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