Find an approach to consolidate the existing validator Map with the new finalized validator index cache

[james-prysm]

💎 Issue

Background

Changes introduced by EIP6110 breaks the existing validator index invariant and consensus layer clients will have to deal with a fact that a validator index becomes fork dependent, i.e. a validator with the same pubkey can have different indexes in different block tree branches.

To solve this, we introduced a new cache that saves the finalized public key to validator index with the usage of it's search laid above the existing validator map.

related PRS:#13943 ,#14146 ,#14173

Description

This puts us in the predicament of maintaining 2 different caches for validators, because we want:

1. To look up based on the point of reference state’s point of view ( the existing validator map that is not reorg safe AKA valMapHandler)
2. To look up based on the canonical chain (new finalized validator index cache)

Ideally we somehow merge these 2 caches or have an elegant way of maintaining outside of the beacon state because of the following statement:

Detailed analysis shows that process_deposit function is the only place requiring a fork dependent (pubkey, index) cache.