Eth2-to-Near-relay: check if clients are syncing

[sept-en]

The Eth2-to-Near-relay should not execute the main run() loop if any of the clients are still syncing.
This may result in a range of issues such as submitting stale blocks or having a NEAR finalized head above the beacon client.

We need to ensure all clients are not syncing during startup if they are sleeping until all clients have finished syncing.
It may be desirable to do these checks periodically during a running client.

• Beacon client has getSyncingStatus.
• Eth1 client has eth_syncing.
• NEAR client has SyncInfo.