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.

peer/test_utils.go

@@ -276,8 +276,12 @@ func createTestPeer(t *testing.T, notifier chainntnfs.ChainNotifier,
 		return nil, nil, err
 	}
 
-	aliceSigner := &mock.SingleSigner{Privkey: aliceKeyPriv}
-	bobSigner := &mock.SingleSigner{Privkey: bobKeyPriv}
+	aliceSigner := input.NewMockSigner(
+		[]*btcec.PrivateKey{aliceKeyPriv}, nil,
+	)
+	bobSigner := input.NewMockSigner(
+		[]*btcec.PrivateKey{bobKeyPriv}, nil,
+	)
 
 	alicePool := lnwallet.NewSigPool(1, aliceSigner)
 	channelAlice, err := lnwallet.NewLightningChannel(