the version value of useragent in the signature should be driven by notation release value

[priteshbandi]

https://github.com/notaryproject/notation-go/blob/0355e8eb494c3e8f9ecb899f12c06ab333739321/signer/signer.go#L24

The semver, currently 1.0.0 should be set based on notation release which means the value needs to be taken as input from caller of sign method.

[priteshbandi]

Adding it as rc2 so that we can triage.

[priteshbandi]

This is partially complete;

1. For SignatureGenerator the fix was done as part of https://github.com/notaryproject/notation-go/pull/178
2. For EnvelopeGenerator fix is pending