Builder pubkey to index cache

We should revisit whether or not we need a pubkey<>index cache for builders. This highly depends on the amount of builders we will have. The tricky part here is that a new builder can created at any time within an epoch (although they are only active after finalization) and indices can be reused, which means the cache can become stale and is not append only like it is the case for validators.

No files, tests, or entry points are named. First locate the builder public-key-to-index lookup and compare it with the validator cache, then trace builder creation, finalization, and index reuse within an epoch. Done means establishing whether a cache is needed and documenting an approach that cannot become stale.

Written by the indexing model from the issue text.