Add support for httpNodeAuth

[knolleary]

Part of FlowFuse/flowfuse#5014

Description

This PR adds support for FF User authentication for the HTTP Node/Dashboard routes.

• It adds the FF Auth plugin. For now, its a copy/paste of the plugin from the nr-launcher repo - was easier than trying to repackage.
• It maintains support for setting httpNodeAuth in the device.yml file - that takes precedence over whatever is configured via the platform
• Bumps device agent to v3.1.0 - this is the feature used on the platform-side PR to detect if these options can be applied to the device.
• Platform-side PR: Add support for httpNodeAuth settings on devices flowfuse#5068

[knolleary]

@Steve-Mcl I've tagged you for a review here. We can discuss details on Monday morning, but would appreciate if you're able to spare an hour on this.

[hardillb]

Approve, but not merging. Needs to be a coordinated release with merging of

FlowFuse/flowfuse#5068

[knolleary]

We can safely merge this one - it is inert until the user enables FF Auth via #5068