Local storage of Signatures

[iamsamirzon]

Summary
The CLI will need access to a local storage
Intended Outcome
Notation CLI implementations allows for this use case
The specfication PR is merged
Additional context
Will update based on finalized design/spec in prior version of alpha release
The implementation work is in #46
Specification work will be in #43

[iamsamirzon]

This is an optimization that we can defer to a RC-2 or later

[iamsamirzon]

Lets talk to @shizhMSFT about this before deciding if this is needed for RC-1

[gokarnm]

• What happens when signatures get deleted in repo?
• What happens when new signature are associated with artifact, will they be pulled?

[iamsamirzon]

The implementation work is in #46
Specification work will be in #43