Reduce usage of `FixedBlobSidecarList` in blob lookups

[realbigsean]

Description

From @dapplion RE: #5583

Reduce our usage of FixedBlobSidecarList in blob lookups. The benefit of this was to more simply merge sets of blob sidecars that might have the same root but different kzg commitments at a specific blob index. This is no longer an issue due to blob inclusion proof veriifcaiton.

WIP PR: realbigsean#39

[realbigsean]

stale