Merge "apply_config_change()" code across local, iroh and tcp helps

[andrewdavidmackenzie]

The code is almost identical, except the sending mechanism at the very end.
Merge them all, either by

• providing a closure to do the sending at the end
• using a trait object that does the sending and pass it in
• using generics to write one code (but then 3 will be generated)

That could unlock a further merge of the three methods in hardware_subscription.rs too....

[andrewdavidmackenzie]

Add a struct for connection, and make it generic over some type, maybe HardwareTarget?, and then use that in subscription?