add new method to test insertability of proposed entries into log

[bobcallaway]

Summary

Given the schema definitions for entry types, there is a fair bit of embedded logic that ensures that entry proposals have the correct mix of properties and not others. For code readability, this PR proposes an intermediate step towards an updated API that is a start towards addressing #1305 by adding a new method to the EntryImpl interface called Insertable, which ensures that the unmarshalled entry has the appropriate fields specified.

• add unit tests for each type

[codecov]

Codecov Report

Merging #1410 (05a14a9) into main (d402c45) will increase coverage by 22.10%.
The diff coverage is 96.46%.

@@             Coverage Diff             @@
##             main    #1410       +/-   ##
===========================================
+ Coverage   43.37%   65.47%   +22.10%     
===========================================
  Files          74       80        +6     
  Lines        7436     7988      +552     
===========================================
+ Hits         3225     5230     +2005     
+ Misses       3873     2106     -1767     
- Partials      338      652      +314     

Flag	Coverage Δ
e2etests	47.25% <8.58%> (?)
unittests	46.44% <96.46%> (+3.07%)	⬆️

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

Impacted Files	Coverage Δ
pkg/types/entries.go	47.69% <0.00%> (+42.60%)	⬆️
pkg/types/test_util.go	0.00% <0.00%> (ø)
pkg/types/alpine/v0.0.1/entry.go	74.33% <100.00%> (+18.56%)	⬆️
pkg/types/cose/v0.0.1/entry.go	69.72% <100.00%> (+14.74%)	⬆️
pkg/types/hashedrekord/v0.0.1/entry.go	76.31% <100.00%> (+22.04%)	⬆️
pkg/types/helm/v0.0.1/entry.go	52.47% <100.00%> (+3.28%)	⬆️
pkg/types/intoto/v0.0.1/entry.go	67.68% <100.00%> (+16.86%)	⬆️
pkg/types/intoto/v0.0.2/entry.go	68.93% <100.00%> (+19.09%)	⬆️
pkg/types/jar/v0.0.1/entry.go	60.86% <100.00%> (+18.97%)	⬆️
pkg/types/rekord/v0.0.1/entry.go	69.18% <100.00%> (+16.22%)	⬆️
... and 3 more

... and 51 files with indirect coverage changes