Refactor Verifiers to return multiple keys

[haydentherapper]

This supports DSSE and intoto types that allow for multiple keys/signatures.

Fixes #1278

Summary

Release Note

Documentation

[haydentherapper]

Only functional change is to return multiple keys for the DSSE and intoto v2 types. Next PR will refactor Identities.

[codecov]

Codecov Report

Merging #1601 (769a719) into main (2bd83da) will increase coverage by 0.07%.
Report is 4 commits behind head on main.
The diff coverage is 66.25%.

@@            Coverage Diff             @@
##             main    #1601      +/-   ##
==========================================
+ Coverage   67.00%   67.07%   +0.07%     
==========================================
  Files          83       83              
  Lines        8389     8441      +52     
==========================================
+ Hits         5621     5662      +41     
- Misses       2092     2101       +9     
- Partials      676      678       +2     

Flag	Coverage Δ
e2etests	48.07% <0.00%> (-0.30%)	⬇️
unittests	47.94% <66.25%> (+0.20%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
pkg/types/dsse/v0.0.1/entry.go	70.62% <0.00%> (-1.78%)	⬇️
pkg/types/entries.go	52.30% <ø> (ø)
pkg/types/rfc3161/v0.0.1/entry.go	65.97% <0.00%> (ø)
pkg/types/test_util.go	0.00% <0.00%> (ø)
pkg/types/hashedrekord/v0.0.1/entry.go	75.25% <50.00%> (-1.06%)	⬇️
pkg/types/intoto/v0.0.1/entry.go	68.67% <50.00%> (-0.67%)	⬇️
pkg/types/jar/v0.0.1/entry.go	60.25% <50.00%> (-0.62%)	⬇️
pkg/types/intoto/v0.0.2/entry.go	66.94% <66.66%> (-0.20%)	⬇️
pkg/types/rekord/v0.0.1/entry.go	70.28% <72.72%> (+1.09%)	⬆️
pkg/types/alpine/v0.0.1/entry.go	75.83% <100.00%> (+2.62%)	⬆️
... and 4 more