Add Forkchoice changes for ePBS

[Subhasish-Behera]

Major changes:
The whole logic is based on the idea that it doesn't matter how you reach the segment-level identifiers. i.e though ePBS introduces a separate envelope object, but the same segment structure can be followed. So the difference between a envelope(full block)/ empty block is which state is read from that block. One of the ways to do that is by following the existing pattern of keeping a chunk of the Store fields inside Chainlink

1. ChainLink dual statefield

  block_state: Option<Arc<BeaconState<P>>>,           // Pre-execution (after beacon block STF)
  execution_payload_state: Option<Arc<BeaconState<P>>>, // Post-execution (after payload STF)

The changed structure of empty/full block pretty much behave as siblings if it is looked from view of part of forkchoice calculation which depends on reading parent information.

• with epbs, to get correct forchoice results, its important to know both which variant the block itself is + which block will it be effecting(for eg: not just the insertion of envelope/empty block have a type but also what it is inserted into has a type i.e the parent block can be full as well as empty)
  so the the most common structure that is going to happen is:

 Segment 0:                ──► [Parent X]
                                │
                                ├──► Segment 1: [Empty A] ──► [Empty B]
                                │                  ▲
                                │                  │
                                │    unfinalized_locations_empty["A"] = (seg:1, pos:0)
                                │
                                └──► Segment 2: [Full A]
                                                 ▲
                                                 │
                                     unfinalized_locations_full["A"] = (seg:2, pos:0)

• both weight propagation purposes as well as the insertion purpose of full/empty block of arrival, the choice of introducing dual maps is much easier than differentiation at unfinalized_block level or using chain_link fields.

• the key currently for both the map is block_root. A previous approach was made to keep unfinalized_locations_full maps key as execution_hash and unfinalized_locations_empty through block_root. But there was significant overhead in terms of number of conversion calla/reverse lookup for hash to root and viceversa. plus as only one variant of full/empty per blockroot can reach till forkchoice, there is ideally no problem with having the same keys.

• a way to move forward with the changes is to look at in which scenarios(insertion into segment, calculation supporting vote, tiebraker , weight propagtion/childern filter/setting ForkChoicePayloadStatus) empty vs full is preferred across spec and current code.