Implement validator custody backfill when CGC increases

[jimmygchen]

See ethereum/consensus-specs#4357

Backfill after CGC increase is optional in the spec, but implementing it provides several benefits:

1. A newly synced node would be able to query the node for recent historical data - potentially useful for L2s
2. Be able to contribute to the overall network sync and make the above easier.
3. The column backfill/import machinery could be useful for downloading checkpoint data columns from peers (Download checkpoint data columns from p2p peers #6837)

TODO:

• Data column import logic Custody backfill sync #7625
• New channel to send messages to SyncManager Custody backfill sync #7625
• Custody backfill service
• Custody backfill metrics

[eserilev]

I'd like to pick this one up