In `SpecVersion`, make `SpecConfig` instance consistent with milestone

[tbenr]

Currently, we always have the highest configured milestone SpecConfig instance associated to each SpecVersion.

This PR makes sure that SpecConfig version correspond to the SpecVersion for a given milestone. For example, for mainnet network, we have SpecConfigPhase0 is returned when we do spec.atSlot(0).getConfig() even when Electra is configured. Similarly for all milestones.

Fixes a counterintuitive behaviour that we frequently forget, causing wasted time, and enables some advanced usage of the SpecConfig (like abstracting away config params that are updated from a milestone to the next, like max blobs)

Documentation

• [ ] I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• [ ] I thought about adding a changelog entry, and added one if I deemed necessary.

[tbenr]

SpecConfigAndParent or something like SpecConfigAndPreviousVersions ?