Add method to get artifact hash for an entry

[haydentherapper]

This can be used to get the entry's artifact hash without requiring unmarshaling the type.

Summary

Release Note

Documentation

[codecov]

Codecov Report

Merging #1777 (6a86e66) into main (2143e33) will increase coverage by 18.83%.
The diff coverage is 37.50%.

@@             Coverage Diff             @@
##             main    #1777       +/-   ##
===========================================
+ Coverage   47.70%   66.53%   +18.83%     
===========================================
  Files          78       90       +12     
  Lines        8182     9048      +866     
===========================================
+ Hits         3903     6020     +2117     
+ Misses       3902     2298     -1604     
- Partials      377      730      +353     

Flag	Coverage Δ
e2etests	48.11% <0.00%> (?)
unittests	47.60% <37.50%> (-0.10%)	⬇️

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

Files	Coverage Δ
pkg/types/entries.go	52.30% <ø> (+47.69%)	⬆️
pkg/types/test_util.go	0.00% <0.00%> (ø)
pkg/types/alpine/v0.0.1/entry.go	74.08% <40.00%> (+16.46%)	⬆️
pkg/types/cose/v0.0.1/entry.go	69.61% <40.00%> (-0.59%)	⬇️
pkg/types/dsse/v0.0.1/entry.go	72.16% <40.00%> (+14.82%)	⬆️
pkg/types/hashedrekord/v0.0.1/entry.go	74.37% <40.00%> (+14.57%)	⬆️
pkg/types/helm/v0.0.1/entry.go	52.94% <40.00%> (-0.25%)	⬇️
pkg/types/intoto/v0.0.1/entry.go	68.14% <40.00%> (+13.05%)	⬆️
pkg/types/intoto/v0.0.2/entry.go	66.57% <40.00%> (+13.46%)	⬆️
pkg/types/jar/v0.0.1/entry.go	59.83% <40.00%> (+16.24%)	⬆️
... and 4 more

... and 51 files with indirect coverage changes