BackFill/Historic Block Sync Counter

[AgeManning]

When a beacon node syncs via a checkpoint sync, it downloads historic blocks in the background.

This is accessible via the /lighthouse/syncing HTTP API endpoint: https://lighthouse-book.sigmaprime.io/api-lighthouse.html#lighthouseeth1syncing

It currenlty shows backfill syncing, but a new update will include the target for the backfill also. From this we can measure a sync distance (i.e number of slots to sync) and therefore display a backfill progress bar like we have for the others.

This is not an urgent feature, but a nice to have at some point in the future.