Match implementation with signature specification

[Pritesh Bandi]

Modify signature implementation to match signature spec

• Remove signedAttributes and use annotations for signed attributes.
• update payload key names.
• Add/update support for version in Artifact manifest and in JWS signature envelope.

[Sajay Antony]

Once this has been merged. We need to update the notation CLI reference.

[Shiwei Zhang]

The third one

Add/update support for version in Artifact manifest and in JWS signature envelope.

needs more discussion and should be in the alpha-2 milestone.

[Shiwei Zhang]

Related issues:

• #17