UI Template: Custom Properties

### Description

This may take some doing to get "right", but the base philosophy is to build off the great work @Steve-Mcl is doing with https://github.com/FlowFuse/node-red-dashboard/pull/634 whereby we could begin to wrap a `ui-template` in a subflow to provide a reusable, configurable component in the Node-RED local palette.

A challenge with this is that often, the `ui-template` is being configured within the Vue component, and doesn't have access to the parent environment variables that would be set at the subflow-level. I see two options here:

### 1. Expose all Environment Variables to Client

_*Security alarm bells ringing*_

### 2. Custom Properties

To circumvent this, we could introduce the concept of "Custom Properties" which would be a generic list of key:value pairs (of varying types) on the `ui-template`. It would then be possible within a subflow to bind these to the subflow env vars, and so have a `ui-template` driven entirely by the subflow properties.

### Epic/Story

_No response_

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

I have provided an initial effort estimate