Node-RED Update Available notification customisation for FlowFuse

When Node-RED spots a new version is available it shows an in-app notification to inform the user.

The notification links them to the Node-RED GitHub release page.

For a FF user, this is not entirely helpful. Their FF env may not yet have updated their stacks to make it available. Nor does a link to the GH release page help the user update their FF-hosted instance.

Via https://github.com/FlowFuse/nr-launcher/pull/560 we have disabled the ui notification to avoid confusion as a short term solution, but we can do better.

Ideally we would have a way to hook into the notification so that FF can inform NR there is a FF-specific update available and to link to FF-specific destination (such as the instance settings page). This will require Node-RED work to provide a standard way of hooking in custom notifications here.