Osiris/background health check

[0xOsiris]

Spawns a Health Service in the background to continuously check that the builder is in sync with the l2 client, and both clients are available.

• If the builder is out of sync (or unresponsive) we set rollup-boost to the PartialContent health status.
• If the l2 client is unresponsive we set rollup-boost to the ServiceUnavailable health status.
• If the builder, and l2 block height match we set rollup-boost to the Healthy status

[0xOsiris]

@teddyknox I changed the health check to assert the unsafe head of the builder is progressing as expected at a threshold of max_unsafe_interval in seconds. An issue with op-geth's unsafe head progression will already be picked by op-conductor, so I think this check was unnecessary. This should be very similar to how op-conductor is asserting unsafe head progression of op-geth as I understand it