feat: re-use EMQX API credentials for the Team Broker configuration

[ppawlowski]

Description

This pull request makes the Team Broker API reuse the EMQX broker's shared bootstrap API key by default, removing the need to manually supply matching credentials. A dedicated credential can still be provided when needed.

Behaviour

• Default: when forge.broker.teamBroker.api.key/forge.broker.teamBroker.api.secret are not set, configuration uses the EMQX default bootstrap key and secret fetched from emqx-config-secrets.api_key_secret (or broker.existingSecret).
• custom: when both api.key and api.secret are set, they are used as a completely separate credential (stored in the flowfuse-secrets resource under the teamBrokerApiSecret); api.key and api.secret must be provided together.
• Team Broker feature continues to require forge.broker.enabled=true.

Related Issue(s)

Closes #923

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