optimistic sync p1 - attest only validated head

[g11tech]

Motivation
There is a high possibility that EL is syncing while CL asks for execution payloads validation (or preparation in case of publish block).

Optimistic sync is for CL to accept the blocks even when the status is not known (tracked via payloadStatusUnknown).
However when the calls to EL for executing payload stops responding as syncing, the payloads status/forkchoice needs to be updated

Checklist:

• Track valid/invalid/syncing and flow the information to the forkChoice/protoarray
• DONOT participate in the if the head's payload status unkown or EL failure
  • block production - if merge not complete still propose a pre-merge block and keep chain going otherwise fail hard
  • attestation,
  • sync committee (sort of done w.r.t what other implementations have been doing i.e. publishing signature but failing on contribution)
    • publish contribution

Considered and Rejected
~~-[ ] Fail hard on invalid executePayload call after EL itself prepared payload via getPayload ~~ publishBlock will fail for now, could be escalated later
To be moved to other Task/PR:

• publish signature ?? in flux about the best way forward, LH impl context : https://discord.com/channels/595666850260713488/892088344438255616/917362756242522143
• Mark Valid , run merge block validation on being valid
• Prune invalid and reorg new head
• Serve the validated head in p2p, req/res, api

Description
UPDATE: working smoothly (producing blocks etc) on a devnet3 node

Closes #3505

[codeclimate]

Code Climate has analyzed commit dd66931 and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	1

View more on Code Climate.

[codecov]

Codecov Report

Merging #3434 (dd66931) into master (5751ee5) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3434      +/-   ##
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- Coverage   37.90%   37.86%   -0.04%     
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  Files         308      308              
  Lines        8129     8144      +15     
  Branches     1248     1252       +4     
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+ Hits         3081     3084       +3     
- Misses       4900     4912      +12     
  Partials      148      148