Support signing attestations using Notary Project

[toddysm]

Notary Project doesn't support signing attestations like for example in-toto attestations. The ask here is to enable Notary Project tooling to sign and store attestations into OCI registries. The following things need to be considered:

• Wrapping attestations in a widely-accepted format (like in-toto)
• Signing using COSE and JWS signature formats
• Support artifact signing in constrained environments (aka local disk signing)

[toddysm]

Also, on the Notary Project GitHub project: notaryproject/notation#1067

It will be helpful to add this functionality to ADO Tasks and other Azure tools that create attestations.