Improve the output for notation plugin

[FeynmanZhou]

What is the areas you would like to add the new feature to?

Notation CLI

Is your feature request related to a problem?

The CAPABILITIES's value [SIGNATURE_GENERATOR.RAW] is not user-friendly from the output of the command notation plugin ls as follows. We need to refine the description.

 notation plugin ls
NAME       DESCRIPTION                                   VERSION   CAPABILITIES                ERROR
azure-kv   Sign artifacts with keys in Azure Key Vault   0.6.0     [SIGNATURE_GENERATOR.RAW]   <nil>

What solution do you propose?

According to the plugin Spec, plugin capability contains four valid values. Revise these capability values to readable information as below would be straightforward for users to understand.

• SIGNATURE_GENERATOR.RAW => Signature generation
• SIGNATURE_GENERATOR.ENVELOPE => Signature envelope generation
• SIGNATURE_VERIFIER.TRUSTED_IDENTITY => Trusted Identity validation
• SIGNATURE_VERIFIER.REVOCATION_CHECK => Certificate chain revocation check

What alternatives have you considered?

N/A

Any additional context?

No response