JWS-based Notation Go Library

[shizhMSFT]

This PR rewrites the previous prototype-level notation go library, and aims for production quality code.

Change logs:

• Removed registry client so that this library only focuses on signing.
  • Closes Move to generic registry and client #8.
• Updated the JWT signature format for prototyping to JWS signature format.
  • Candidate specification: Signature Format Specification specifications#93.
  • Closes Implement updated signature format - JWS candidate specification  #9.
  • The JWS implementation is based on golang-jwt/jwt v4.
    • Plugable signing and verification methods.
  • The resulted JWT is reserialized to / from JWS JSON serialization by code re-written from shizhMSFT/go-jwsutil.
  • New notation library interfaces.
• Integrated timestamping to the JWS signature format.
  • Related discussion on expiry: [Draft] Trust Policy and Trust Store Configuration specifications#98
  • Developed package crypto/timestamp for fetching and verifing timestamp signatures from TSAs.
    • Code is re-written from shizhMSFT/go-timestamp.
• Resolves Tracking issue for pending feedbacks/Issues in PR5 #10.
• Tested by unit tests.
• Documented all the packages.
• Added a Makefile for easier development.
• Added GitHub actions for continuous testing.
• Added dependency bot for dependency updates.

[priteshbandi]

LGTM, some minor comments

[NiazFK]

LGTM

[priteshbandi]

LGTM