feat: make health check configurable

[matthewelwell]

This PR looks to solve #120.

It does so by making certain aspects of the health check configurable, with the bear minimum health check being to confirm that the environment document has been updated at some point.

[matthewelwell]

My main concern with the current approach in this PR is how health checks will handle start up when environment_service.last_updated is expected to be null. I think this is fine, although thinking about it, the behaviour would have been the same before because we would have just suppressed the TypeError and returned status_code=500.

[gagantrivedi]

My main concern with the current approach in this PR is how health checks will handle start up when environment_service.last_updated is expected to be null. I think this is fine, although thinking about it, the behaviour would have been the same before because we would have just suppressed the TypeError and returned status_code=500.

Yeah, I think that should be the responsibility of the service to give some grace period for the proxy to become healthy