`SkippedByTrustPolicy` from JSON output of `notation verify` is not documented

[shizhMSFT]

What is the areas you experience the issue in?

Notation CLI

What is not working as expected?

The result status SkippedByTrustPolicy from the JSON output of notation verify is not documented.

Although the follow-up scripts or program can determine the verification status by the return code, developers do not know there is a value SkippedByTrustPolicy besides Success unless they read through the source code.

What did you expect to happen?

Have the JSON output of notation verify documented, including the enums of the result status.

[byronchien]

if a "reason" field is being added for this issue, then instead of having "SkippedByTrustPolicy" as the result, will just use "Skipped" and mention the trust policy in the reason. Will still add documentation for the different result values.

[sajayantony]

Can we also improve the output of -v

I think we can improve the user experience by highliging key areas that are important and removing unnecessary golang masrhalling info. /cc @shizhMSFT

signer@3344ae675ce7:/notation-demos$ notation verify -v $IMAGE
INFO Using the referrers tag schema
INFO Reference sha256:934e37d5c1029186129234a1af1d46b90a9d6b14c02276536e6e9b2ec98db3a9 resolved to manifest descriptor: {MediaType:application/vnd.oci.image.manifest.v1+json Digest:sha256:934e37d5c1029186129234a1af1d46b90a9d6b14c02276536e6e9b2ec98db3a9 Size:507 URLs:[] Annotations:map[] Data:[] Platform:<nil> ArtifactType:}
INFO Checking whether signature verification should be skipped or not
INFO Trust policy configuration: &{Name:notation-demos-images RegistryScopes:[localhost:5000/hello] SignatureVerification:{VerificationLevel:strict Override:map[]} TrustStores:[ca:notation-demos.io] TrustedIdentities:[*]}
INFO Check over. Trust policy is not configured to skip signature verification
INFO Processing signature with manifest mediaType: application/vnd.oci.image.manifest.v1+json and digest: sha256:8d11c5ea366ac877d2a8fd8505a2445a406710cddb91504f0156238ceff45195
INFO Trust policy configuration: &{Name:notation-demos-images RegistryScopes:[localhost:5000/hello] SignatureVerification:{VerificationLevel:strict Override:map[]} TrustStores:[ca:notation-demos.io] TrustedIdentities:[*]}
Successfully verified signature for localhost:5000/hello@sha256:934e37d5c1029186129234a1af1d46b90a9d6b14c02276536e6e9b2ec98db3a9

[yizha1]

Closed by #551