Add healthcheck support for JetStream

[vivianrwu]

This PR adds the healthcheck support endpoint for JetStream.
This covers checking the model server is alive based on the self.live field here - https://github.com/google/JetStream/blob/e19a7906d8cdf1cae658a4c7c4f6f516aade49f9/jetstream/core/orchestrator.py#L377

The healthcheck endpoint can be invoked via the following:

healthcheck_request = jetstream_pb2.HealthCheckRequest()
healthcheck_response = stub.HealthCheck(healthcheck_request)
healthcheck_response = await healthcheck_response

healthcheck_response: {
  is_live: bool
}

Test coverage can be found under test_server.py once the server is started.
This functionality will be useful for model server liveness and readiness checks.

[liurupeng]

@vivianrwu could you add a description about why we are adding this PR? thanks! (as well as the test steps

[JoeZijunZhou]

LGTM! After fixing the CI checks, we are good to go!

[FanhaiLu1]

can you share where driver set live status?

[vivianrwu]

Yes, it is set here - https://github.com/google/JetStream/blob/e19a7906d8cdf1cae658a4c7c4f6f516aade49f9/jetstream/core/orchestrator.py#L377

[FanhaiLu1]

Thanks for sharing it!