The proxy container defaults `--management-url` to
`https://api.netbird.io`, so on self-hosted NetBird the proxy talks to
SaaS and rejects the setup key as invalid.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added support for configuring a management URL for the cluster proxy
service.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
