add get_block_header method

hive.blocks has been expanded by:

• transaction_merklee_root
• extensions (stored as json, usually this field is empty, small impact on database size)
• witness_signature
• signing_key - signing key of witness, this is not used in get_block_header, however when we add also method get_block - its required. All other data for get_block are already in database. There is also extensions for transaction, but this field is hardcoded as empty, ie flat_set<static_variant<void_t>>