Define where a msg is sent by 'project link nodes' using msg.topic

### Description

It would be useful if setting msg.topic could route a msg from a project out node to a multiple different project in nodes.

Example: https://d.pr/i/we4H5R

GS edit:
As a user, I want to define topics dynamically in project link nodes, so that I can easily create a UNS with a minimal number of nodes.

Creating topics dynamically, where the topic sent from a project node is defined using some variable that picks up the appropriate content that is sent through it, would allow the user to put a single project node in place instead of one node per precise topic that is sent through the project node. This would make the development of a UNS with FlowFuse much simpler.

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

I have provided an initial effort estimate