Cleanup block header fields

[Longarithm]

There are many fields in block header (BlockHeader::BlockHeaderV5, as of 5 Oct 2023), which are either not used, or their usecase is unclear. This leads to unclear block validation logic, see https://github.com/near/nearcore-private/issues/38. The good practice is to sign as less data as possible, to minimize vulnerability angles, in cases when malicious actor sends invalid data.

We can take analysis from #9618 and:

• remove obviously unused fields, like challenges_*, chunk_tx_root.
• specify use case of fields like block_ordinal, epoch_sync_data_hash , prev_validator_proposals. Document why we need to sign these fields.
• group necessary fields by usecases to simplify reading, for example:
  • fields for runtime: gas price - to deduct balances correctly, random value - to use randomness in contracts, etc.
  • fields for epoch manager: validator_proposals, chunk_mask, latest_protocol_version
  • consensus: approvals, *_final_block, ...
  • something else?