refactor: use DSSE wrappers from sigstore

[tri-adam]

Leverage DSSE wrappers from github.com/sigstore/sigstore, as they line up a little well with the Signer/Verifier we're using.

[codecov]

Codecov Report

Attention: Patch coverage is 85.10638% with 7 lines in your changes missing coverage. Please review.

Project coverage is 62.59%. Comparing base (d63ee14) to head (fc6a037).

Files	Patch %	Lines
pkg/integrity/dsse.go	84.44%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #366   +/-   ##
=======================================
  Coverage   62.59%   62.59%           
=======================================
  Files          37       37           
  Lines        3280     3240   -40     
=======================================
- Hits         2053     2028   -25     
+ Misses       1056     1046   -10     
+ Partials      171      166    -5     

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[dtrudg]

LGTM