Expose the endpoint for health checking

[longfin]

The bridge relies heavily on the state of two external systems: 9c mainnet and Ethereum mainnet, which is why synchronization is so important. In other words, even if the bridge process is running without errors, it is meaningless if this sync cannot be maintained.

In order to better understand this synchronization status from the outside, if there is an API that exposes the status of these external systems that the bridge is currently observing (i.e., the index of the latest block), it can be used for health check, etc.