Implementing signature filtering

[yizha1]

Summary

Implementing signature filtering according to "Filter signature artifact manifest" documented in https://github.com/notaryproject/notaryproject/blob/main/signing-and-verification-workflow.md#verification-steps

Intended Outcome

Signature filtering works as described in "Filter signature artifact manifest"

Additional context

[FeynmanZhou]

This implementation request has been added to the Notary planning board. Let's see who will be interested in picking it up.

[shizhMSFT]

oras-go rc.1 is released. We can start work this issue.

[dtzar]

Agreed we can move this past RC-1 on the community call 7/21.