various REST serialization cleanups

[etan-status]

• delete unused version constants
• move EpochParticipationFlags serialization to other serializers
• move JustificationBits serialization after base types
• add eth-consensus-version for JSON in uniform way
• add sszResponsePlain for pre-encoded SSZ data
• use withState in jsonResponseState
• add missing section comments
• use GraffitiBytes.init reader instead of hexToByteArray
• rm unused Version/ForkDigest/DomainType/GraffitiBytes reader
• rm unused dump function for KeystoresAndSlashingProtection
• extend REST test with checks for Eth-Consensus-Version

[github-actions[bot]]

Unit Test Results

       9 files  +       6     660 suites  +440   19m 43s :stopwatch: + 13m 31s 1 982 tests +   148  1 835 :heavy_check_mark: +     20  147 :zzz: +128  0 :x: ±0  8 060 runs  +5 372  7 889 :heavy_check_mark: +5 220  171 :zzz: +152  0 :x: ±0 

Results for commit fb618501. ± Comparison against base commit 23007eac.

:recycle: This comment has been updated with latest results.

[etan-status]

Better to split this one into multiple PRs.