Call `eth_syncing` to set the state of the Engine API.

[mpaulucci]

We want to know if the Engine API is online or offline.

For example, lighthouse has the upcheck function (https://github.com/sigp/lighthouse/blob/441fc1691b69f9edc4bbdc6665f3efab16265c9b/beacon_node/execution_layer/src/engines.rs#L248) that periodically calls the execution api to get the current state. There are four internal states: Synced, Offline, Syncing, AuthFailed, but are exposed only as Online & Offline.

Need to dig deeper to investigate what are the implication if the Execution client is offline