Enable WebSocket Affinity in the device agent

[hardillb]

part of FlowFuse/flowfuse#2514

Description

As part of work to enable horizontal scaling of the Forge App, this allows the Device agent to use a cookie to bind to a specific instance (using Nginx Affinity mapping to start with).

Reuses the affinity cookie if present, shares the affinity cookie with the forge platform if available.

Related Issue(s)

FlowFuse/flowfuse#2514

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • [ ] Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• [ ] Backport needed? -> add the backport label
• [ ] Includes a DB migration? -> add the area:migration label

[hardillb]

Need to work out how to test this

[hardillb]

I've used the default Cookie name here, we may want to set it to a FF specific value.