multi: update models.ChannelAuthProof with v2 field

Also update it to more closely match the persisted version which has the
v1 and v2 only fields as optional.

Refactor ChannelAuthProof to support both v1 and v2 channel
announcements:

- Add Version field to distinguish v1 from v2 proofs
- Wrap v1-specific fields (NodeSig1/2, BitcoinSig1/2) in fn.Option
  since v2 doesn't use them
- Add optional Signature field for v2's single schnorr signature
- Add constructor functions NewV1ChannelAuthProof and
  NewV2ChannelAuthProof to enforce correct initialization
- Add getter methods (NodeSig1(), BitcoinSig1(), etc.) that safely
  unwrap options, returning empty slices when not present

The IsEmpty() check is updated to handle both versions correctly.
Both stores validate v1-only for now.

routing/pathfind_test.go

@@ -99,12 +99,12 @@ var (
 	_             = testSScalar.SetByteSlice(testSBytes)
 	testSig       = ecdsa.NewSignature(testRScalar, testSScalar)
 
-	testAuthProof = models.ChannelAuthProof{
-		NodeSig1Bytes:    testSig.Serialize(),
-		NodeSig2Bytes:    testSig.Serialize(),
-		BitcoinSig1Bytes: testSig.Serialize(),
-		BitcoinSig2Bytes: testSig.Serialize(),
-	}
+	testAuthProof = *models.NewV1ChannelAuthProof(
+		testSig.Serialize(),
+		testSig.Serialize(),
+		testSig.Serialize(),
+		testSig.Serialize(),
+	)
 )
 
 // noProbabilitySource is used in testing to return the same probability 1 for