Checkpoint sync from states newer than finalization

[ethDreamer]

Description

One of the main edge cases to worry about post-deneb is if the chain takes longer than the data availability boundary to finalize. In such a case, nodes must be able to weak subjectivity sync off of non-finalized states that are still within the data availability boundary.

Present Behaviour

Currently, lighthouse will only request a finalized state from the remote node. This is bad if we run into this edge case.

Expected Behaviour

Lighthouse should be able to accept an anchor state id (finalized, justified, slot, root) from which to sync from. Regardless of whether or not the state is finalized, lighthouse should refuse to sync from a chain that conflicts with the anchor state.

It's not as simple as just treating the anchor state as finalized because lighthouse will then believe it is ahead of all of its peers and won't be able to sync because it filters out peers with lower finalized states.