feat: Add Valkey cache for scaling

[hardillb]

fixes #699

Description

Adds a Valkey instance if more than 2 Forge app instances are configured.

Related Issue(s)

#699
FlowFuse/flowfuse#3642

Checklist

• 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
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[hardillb]

Need to decide if we are happy with no auth to start with here?

[ppawlowski]

Need to decide if we are happy with no auth to start with here?

If we are not going to store fragile data in the cache, I'm ok with no auth + network policy that will allow connections to the cache from the Forge deployment only.

[hardillb]

@ppawlowski i rebased this on main to resolve some merge conflicts, can you double check the Chart.lock and anything else please.

[hardillb]

@ppawlowski I'm going to merge this, we should be aware this will deploy Valkey (even with a single Forge app instance) when the CloudProject helm version releases are updated.

[ppawlowski]

I'm going to merge this

Go for it.

[ppawlowski]

I've added a valkey values template guard to ensure that proper valkey configuration is always present, depending on the provided values. Feel free to accept or reject it - it does not affect the overall feature implementation.
I'm fine with the latest changes, merge anytime.