Refactor rewards api to prepare for lodestar-z integration

[twoeths]

Problem description

right now we couple typescript state-transition package in beacon-node in order to implement rewards api

lodestar/packages/beacon-node/src/chain/chain.ts

Line 1319 in 889b1c4

preState = processSlots(preState, block.slot); // Dial preState's slot to block.slot

specifically it call processSlots() and beforeProcessEpoch() and it access some properties of CachedBeaconState there

Solution description

need to refactor in a way that same beacon-node code can also work with lodestar-z state-transition
need to open an issue for it in that case

this is part of #8650

Additional context

No response