Need to wait for API to initialize when backend starts

[javierluraschi]

Currently we have a pretty ugly workaround which is to call eval/ multiple times when it fails. In order to remove this, we need h9 start to finish execution until the runtime is ready, otherwise, there is no way for the client to know if an eval/ request is an actual error or just failed due to slow initialization.

One way to support this is to add a /health api to plumber / fastapi which returns OK. The rust server will have to it multiple times during h9 start until it succeeds, if it doesn't, it would fail.

Once we have this, we can remove this workaround from the client.