Add a metric for optimistic sync status

[michaelsproul]

We should add a metric tracking the optimistic sync status of the head block. Something like:

head_block_execution_verified 0|1

where:

• 0 represents optimistic sync (head block is Syncing in fork choice)
• 1 represents proper sync (head block is Valid in fork choice)

I think phrasing it in the positive (1 = good) makes sense, because this maps onto a nice big "SYNCED" metric on a Grafana dashboard.

[chong-he]

I would like to pick this up

[michaelsproul]

Perfect, thanks CK!