Import sync committee messages into op pool

[michaelsproul]

Description

We sweep all unaggregated attestations from the naive aggregation pool into the op pool before proposing a block. We could do similarly for sync committee messages.

This would provide some way of getting messages on chain when the sync aggregation system fails, which is expected occasionally due to the probabilistic nature of determining aggregator status (although it should be a lot better with the # of target aggregators bumped from 4 to 16).

This would also require us to attempt aggregation of any compatible sync contributions as they are added to the pool, as raised by @paulhauner here: https://github.com/sigp/lighthouse/pull/2321/files#r648051400

[tthebst]

I will look into this. Sounds interesting