Input node: Send Topic with msg throughput

### Description

When using Input nodes to populate a flow memory for inputs of a form, it was advised that the Topic value should be used to add values to the flow memory location (using a change node to set the payload value to flow.formSubmission[_client.user.username][msg.topic]. This works well.
Until you send a message to the input node that has no or a different Topic. In which case the value sent to the Input node is displayed in the Dashboard, but the value is not saved in the flow memory location (because the Topic value is missing).

The obvious workaround for this is to have another Change node after the Input node that sets the Topic (again, as it is already set as a string value in the Input node). But if I have to use another Change node to make sure the Topic is correct, why does the option exist in the Input node?
I would like to propose that the Topic that is set in the Input node is also sent with the outgoing message when msg passthrough is enabled and if no Topic is specified in the incoming message.

This would eliminate the need for an additional Change node.
And make the node function as described in the properties:
When changed (which it obviously does when sending an input value message) send:
* Payload : current value
* Topic: misc setting

I've added this as a FR, however because of the properties description it might be more of a bug than a FR.

### Have you provided an initial effort estimate for this issue?

I am no FlowFuse team member