Allow ui_update for output messages

### Description

Hi guys,

I always try to keep my flows as simple and small as possible.

However when I want to wire the output of a ui node to the input of another ui node, I need to add Change nodes all over the place:


The reason why I need those change nodes is because:
+ In the button node I only can specify the `msg.payload` and `msg.topic` of the output message.
+ In my new hls node I expect the data to be in `msg.ui_update`.

I really like Change nodes, but in this case they don't add something useful to my flows. They only add extra unnecessary visual complexity. The following flow is what I would like to achieve:


Since `msg.ui_update` is a standard way of working in this new dashboard, I was wondering if is ok for me to add an extra field to the config screen some nodes (e.g. button node):


Thanks!
Bart

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

I am no FlowFuse team member

Start by reviewing the button node's config screen and its output-message path, which the issue identifies as the affected entry points. Trace how output fields are currently selected and compare that with the ui_update data expected by the HLS node. Done means the requested output configuration works without intermediary Change nodes and does not regress existing button outputs.

Written by the indexing model from the issue text.