Update node health endpoint

[michaelsproul]

Description

As suggested by @jmcruz1983 here we should update the /eth/v1/node/health endpoint to account for the execution node's sync status.

I think we should change the semantics to return syncing if:

let unhealthy = syncing || el_offline || optimistic

Currently it's just checking the BN's sync status.

Additional Info

Related to #4291, which will add the el_offline field.

[eserilev]

i'd like to pick this one up

[michaelsproul]

thanks @eserilev! let us know if you need any pointers

the EL offline methods added in #4291 can probably be used for the el_offline part