Setup Fly.io checks for spark-stats service

[bajtos]

ATM, we don't tell Fly.io how to check whether the HTTP server is up and accepting requests.

We should configure checks in stats/fly.toml file.

Docs:

https://fly.io/docs/reference/configuration/#the-checks-section

While we can draw inspiration from spark-api fly.toml, I think it would be better to design the checks from scratch to use HTTP checks. spark-api is using TCP checks.