Add /health endpoint

[drogus]

Description of Changes

This PR adds a health endpoint with some potentailly usefeul information like:

• package name (so we can distinguish between standalone and cloud)
• package version
• if the current node is schedulable
• a list of nodes in the cluster

API and ABI breaking changes

No breakage, the endpoint didn't exist in standalone before.

Expected complexity level and risk

1

Testing

• Local test
• Start the server, check if the /health endpoint responds with 200

[kurtismullins]

It looks good to me but I don't know enough about our internal system (e.g get_nodes()) to be a good reviewer of this.

I think that as long as it actually represents the database being healthy, its better than what we have now!