Update Plug-in spec to support multiple signature formats

[SteveLasker]

To support multiple signature formats (#145) the plug-in spec will need updates to support multiple signature/envelope formats. Specifically, JWT and COSE.

A desired outcome is the plug-in spec would specify a named parameter for what signature format is requested, and the plug-in APIs wouldn't need changing.

[sajayantony]

/cc @gokarnm

[gokarnm]

The spec already supports multiple envelope format using signatureEnvelopeType, in the Signature Envelope Generator interface. Let me know if any other changes are required.

{
  "contract-version" : <major-version.minor-version>,
  
  // Complete key definition from /notation/config.JSON /signingKeys/keys with matching key name
  "keyDefinition" : <key definition>, 

  "payload" : <Base64 encoded payload to be signed>,
  
  // The type of payload - currently a descriptor
  "payloadType" : "application/vnd.oci.descriptor.v1+json",
  
  // The expected response signature envelope
  "signatureEnvelopeType" : "application/vnd.cncf.notary.v2.jws.v1"
}

[SteveLasker]

@shizhMSFT, can you confirm your question is answered and adding code doesn't change the plug-in spec?