feat(mev-rs): Refactor and Bench Validator Registry State Extension

[refcell]

Description

Refactors the validator registry state extension and benchmarks distributive vs grouped iterator map construction.

Follow-on work should be done to explore the usage of bijective mappings and more high level architectural enhancements.

[ralexstokes]

nice! would like to see numbers from the benchmark

the comment about "loading more efficiently" refers to grabbing validator updates from the beacon chain however, and not about the internals of the validator registry here

right now, it just grabs the entire val set on each update, when it could be far more clever around just grabbing updates to those validators that had any change in their state across an epoch (for example)