input+wallet: extract musig2 session management into new module

In this commit, we extract the musig2 session management into a new
module. This allows us to re-use the session logic elsewhere in unit
tests so we don't need to instantiate the entire wallet.

watchtower/wtmock/signer.go

@@ -71,7 +71,8 @@ func (s *MockSigner) ComputeInputScript(tx *wire.MsgTx,
 // submitted as well to reduce the number of method calls necessary later on.
 func (s *MockSigner) MuSig2CreateSession(input.MuSig2Version,
 	keychain.KeyLocator, []*btcec.PublicKey, *input.MuSig2Tweaks,
-	[][musig2.PubNonceSize]byte) (*input.MuSig2SessionInfo, error) {
+	[][musig2.PubNonceSize]byte,
+	...musig2.SessionOption) (*input.MuSig2SessionInfo, error) {
 
 	return nil, nil
 }