update: Package notation refactoring

[patrickzheng200]

PR review order: #190 -> #192 -> This PR -> #186.

This PR refactors the notation package, files intended to be reviewed in this PR:

1. files in notation folder
2. internal/mock

internal/common, verification/trustpolicy and verification/truststore pakcages shall be reviewed in #192 already. Therefore, they can be skipped in this PR.

Files in internal/mock_origin, internal/registry, and internal/notation.go are there just for not breaking the old dependencies. They will be removed once the refactoring is done.

[codecov-commenter]

Codecov Report

Merging #191 (23a6feb) into main (f9c0b87) will decrease coverage by 2.76%.
The diff coverage is 29.41%.

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
- Coverage   80.10%   77.34%   -2.77%     
==========================================
  Files          31       33       +2     
  Lines        2071     2189     +118     
==========================================
+ Hits         1659     1693      +34     
- Misses        325      405      +80     
- Partials       87       91       +4     

Impacted Files	Coverage Δ
errors.go	0.00% <0.00%> (ø)
signature/envelope.go	100.00% <ø> (ø)
signature/plugin.go	79.25% <ø> (ø)
signature/signer.go	38.23% <ø> (ø)
verification/verifier.go	83.33% <ø> (ø)
notation.go	34.69% <34.69%> (ø)
registry/repository.go	33.84% <100.00%> (ø)

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[shizhMSFT]

LGTM

[shizhMSFT]

LGTM

[priteshbandi]

LGTM with comments addressed.

[shizhMSFT]

LGTM