input: add new Preimage method to input.Input

In this commit, we add a new method to obtain an option of a preimage to
the input.Input struct. This is useful for callers that have an Input,
and want to optionally obtain the preimage.

input/mocks.go

@@ -140,6 +140,17 @@ func (m *MockInput) ResolutionBlob() fn.Option[tlv.Blob] {
 	return info.(fn.Option[tlv.Blob])
 }
 
+func (m *MockInput) Preimage() fn.Option[lntypes.Preimage] {
+	args := m.Called()
+
+	info := args.Get(0)
+	if info == nil {
+		return fn.None[lntypes.Preimage]()
+	}
+
+	return info.(fn.Option[lntypes.Preimage])
+}
+
 // MockWitnessType implements the `WitnessType` interface and is used by other
 // packages for mock testing.
 type MockWitnessType struct {