reuse DSSE signature wrappers instead of a local copy

[bobcallaway]

This removes some duplicate code wrapping a DSSE signer/verifier which already existed in sigstore/sigstore.

Signed-off-by: Bob Callaway bcallaway@google.com

[codecov-commenter]

Codecov Report

Merging #912 (c9f3116) into main (fb4ed40) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #912      +/-   ##
==========================================
- Coverage   48.16%   48.16%   -0.01%     
==========================================
  Files          62       62              
  Lines        5427     5398      -29     
==========================================
- Hits         2614     2600      -14     
+ Misses       2525     2515      -10     
+ Partials      288      283       -5     

Impacted Files	Coverage Δ
pkg/types/intoto/v0.0.1/entry.go	56.09% <100.00%> (+3.10%)	⬆️
pkg/types/alpine/v0.0.1/entry.go	55.46% <0.00%> (-1.27%)	⬇️
pkg/types/rekord/v0.0.1/entry.go	48.82% <0.00%> (-0.67%)	⬇️
pkg/types/helm/v0.0.1/entry.go	48.91% <0.00%> (ø)

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