New lab liveness endpoint

[lukashornych]

In the evitaDB's lab api, there will be new endpoint /lab/liveness which will return 200 or otherwise to signalize if all required APIs are accessible and ready. We will use this in evitaLab instead of fetching server status from gRPC API. The new endpoint will also internally probe the gRPC API because it is mandatory to be enabled and alive for evitaLab to function.

This will allow us to:

• verify that the user is using proxied lab URL and not gRPC API URL directly without complex verification
• check that gRPC is enabled

[lukashornych]

Related evitaLab issue lukashornych/evitalab#207