Update plugin spec to support extended attributes

[yizha1]

    I raised [this PR](https://github.com/notaryproject/notation-go/pull/188/files) to unblock JWS signatures. Please raise a PR for spec updates once you have an idea on how verification plugins needs to deal with COSE extended attributes

Originally posted by @rgnote in notaryproject/notation-core-go#88 (comment)

This issue is to address how plugin spec can support extended attributes for different signature formats.

[yizha1]

This issue is still valid and could introduce a breaking change, so move it to the future milestone and label it with v2.