Add SignerDID to jwt.Decoded

[tomdaffurn]

jws.Decoded has a field to expose the signer's DID. jwt.Decoded should add the same field. This allows verifiers to make assertions about the JWT issuer.

jwt/jwt.go

// Decoded is a compact JWT decoded into its parts
type Decoded struct {
	Header    Header
	Claims    Claims
	Signature []byte
	Parts     []string
+	SignerDID _did.DID
}