Standardizing the spec endpoint

[mcdee]

The /eth/v1/config/spec endpoint should in theory return the same information from each beacon node client, however in practice it does not. The recent issue with holesky has highlighted the benefit of having a standard configuration, so we should address the inconsistencies in this endpoint.

Addressing the inconsistencies is likely to have multiple facets, for example:

• ensure that existing clients adhere to the spec as-is
• tighten up the spec where it is open to interpretation
• define what to do with values that are not specified in an existing repo (e.g. terminal total difficulty for sepolia)
• how to handle speculative values (e.g. value for future forks which will turn up at different times in different codebases)

This issue is here to be used as a reference point for all such changes to specification and implementations.