Add ability to swap crypto implementation

[shizhMSFT]

The v1.0.0-rc.1 version of go-cose computes the digest of CBOR-encoded signature structure using the golang built-in hash implementations regardless of the implementation of Signer and Verifier.

If someone wants to change the implementation of golang built-in crypto packages (e.g. use openssl based implementation) just for go-cose, they can only change for Signer and Verifier but not the hash implementations.

Proposal: Instead of passing digest to Sign() and Verify(), we should pass content (i.e. raw payload). Thus it matches RFC 8152 section 8.

The signature functions for this scheme are:

  signature = Sign(message content, key)

  valid = Verification(message content, key, signature)

It also addresses the weird case that Ed25519 passes CBOR-encoded signature structure instead of digest to Signer as Ed25519 requires the content to be processed twice.