Add support for identifying authentication messages

The pgprotocol overloads 'p' messages with PasswordMessage,
SASLInitialResponse, SASLResponse, and GSSResponse. This patch allows
contextual identification of the message by setting the authType in the
frontend and then setting this value in the backend when a
AuthenticationResponseMessage is received.

pgproto3.go

@@ -27,6 +27,11 @@ type BackendMessage interface {
 	Backend() // no-op method to distinguish frontend from backend methods
 }
 
+type AuthenticationResponseMessage interface {
+	BackendMessage
+	AuthenticationResponse() // no-op method to distinguish authentication responses
+}
+
 type invalidMessageLenErr struct {
 	messageType string
 	expectedLen int