### What problem does this PR solve?

Adds a new Kubernetes Service resource to the Helm chart which
specifically targets the RAGFlow API. This feature useful for cases
where you want to expose the RAGFlow HTTP API separately from the web
interface, for example if RAGFlow is running behind an authenticating
proxy it allows a route to bypass the proxy (e.g. by defining a separate
ingress resource which forwards to the separate API-only k8s service
added here) to provide RAGFlow API access. This is still secure since
API access is already authenticated by API keys inside RAGFlow itself.

### Type of change

- [X] New Feature (non-breaking change which adds functionality)