lnwallet: add new aux resolver interface

This will be used by external callers to modify the way we resolve
contracts on chain. For a given contract, we'll store an extra "blob",
that will later be presented during the sweeping phase.

lnwallet/mock.go

@@ -493,3 +493,14 @@ func (a *MockAuxSigner) VerifySecondLevelSigs(chanState AuxChanState,
 
 	return args.Error(0)
 }
+
+type MockAuxContractResolver struct{}
+
+// ResolveContract is called to resolve a contract that needs
+// additional information to resolve properly. If no extra information
+// is required, a nil Result error is returned.
+func (*MockAuxContractResolver) ResolveContract(
+	ResolutionReq) fn.Result[tlv.Blob] {
+
+	return fn.Ok[tlv.Blob](nil)
+}