lnwire: add new musig2 partial signature type

In this commit, we add the new types that'll house musig signatures with
and without their nonces. We send the nonce along with the sig
everywhere but the co-op close flow.

lnwire/writer_test.go

@@ -160,7 +160,9 @@ func TestWriteSig(t *testing.T) {
 
 func TestWriteSigs(t *testing.T) {
 	buf := new(bytes.Buffer)
-	sig1, sig2, sig3 := Sig{bytes: [64]byte{1}}, Sig{bytes: [64]byte{2}}, Sig{bytes: [64]byte{3}}
+	sig1 := Sig{bytes: [64]byte{1}}
+	sig2 := Sig{bytes: [64]byte{2}}
+	sig3 := Sig{bytes: [64]byte{3}}
 	data := []Sig{sig1, sig2, sig3}
 
 	// First two bytes encode the length of the slice.