Restore runtime configurable EthSpec

[realbigsean]

Description

In a bunch of tests we set the EthSpec at runtime, and this is no longer possible because FIELD_ELEMENTS_PER_BLOB is required at compile time in the c-kzg lib. Michael's raised an issue here for how to resolve it: ethereum/c-kzg-4844#312

When it's resolved, we have some spec-minimal vs spec-mainnet feature gating in tests that we will be able to remove.

[realbigsean]

Since ethereum/c-kzg-4844#312 was merged, we're now in a place where we can work out this:

When it's resolved, we have some spec-minimal vs spec-mainnet feature gating in tests that we will be able to remove.

I can pick up this part because I have a pretty good idea of where we did this type of feature gating. So I'm gonna re-assign this issue to me. Thanks for figuring this out michael! This was a massive pain point.

[realbigsean]

resolved in #4468