Status endpoint to tell if the instance is "active"

[ThomasDalla]

When using the multiinstance feature, it would be great to have a /status endpoint that tells whether or not the instance is active.
It can be a simple header only call (with different HTTP Codes returned) or one with a body like { "active": true } / { "status": "active" }.

With such endpoint, we could setup a simple haproxy instance that checks that endpoint on both instance and route to the active one.
Then the builder endpoint on the beacon nodes should be set to that haproxy instance so that it always routes to the active one.