restore get_block_header_state endpoint for usage both pre and post Savanna activation

[spoonincode]

In the past, get_block_header_state returned the pre-Savanna (legacy) block header state. Parts of this response are incompatible with the internals of the new Savanna block state. Originally the plan was to simply remove the get_block_header_state endpoint, but some versions of eosjs (including the latest one, 22.1.0, apparently downloaded some 17k times in the last week) require operation of this endpoint given certain values of blocksBehind otherwise eosjs will be unable to push a transaction.

So instead, restore the endpoint with some behavioral differences. No matter whether Savanna has been activated or not, get_block_header_state in Spring 1.0 will return a response containing all fields but with only block_num, id, header, and additional_signatures filled out. Other fields will still exist but will be empty or zero. Additionally, the endpoint will consider both reversible and irreversible blocks. This latter tweak helps guard against a race condition in eosjs between calling get_info and then get_block_header_state when blocksBehind is a low number such as 2 or 3.

An example response with the limited filled fields looks something like,

{
  "block_num": 40660,
  "dpos_proposed_irreversible_blocknum": 0,
  "dpos_irreversible_blocknum": 0,
  "active_schedule": {
    "version": 0,
    "producers": []
  },
  "blockroot_merkle": {
    "_active_nodes": [],
    "_node_count": 0
  },
  "producer_to_last_produced": [],
  "producer_to_last_implied_irb": [],
  "valid_block_signing_authority": [
    0,
    {
      "threshold": 0,
      "keys": []
    }
  ],
  "confirm_count": [],
  "id": "00009ed4aa662f3d7e96d88061e4741692b433fe783befc6c3cb6c6a40c5955a",
  "header": {
    "timestamp": "2024-03-19T02:39:40.000",
    "producer": "inita",
    "confirmed": 0,
    "previous": "00009ed38d8d4c103ce5ced75558d6ac0f4d2ac4b63cf964feeb6d8bce600ade",
    "transaction_mroot": "0000000000000000000000000000000000000000000000000000000000000000",
    "action_mroot": "b179283d2264aa663cb669924bbff2f33e81a7ed71dcb465342673602605a1f1",
    "schedule_version": 2,
    "header_extensions": [
      [
        2,
        "d39e0000010000"
      ]
    ],
    "producer_signature": "SIG_K1_KgufADyFuHdBwT6VGBVdrnhVs6dakZdXp4qr5NgJFU7orfXbFi9eVc7NvjrjvUyL79SXfMjgyzW7cVGfQW8iy1CbjStENZ"
  },
  "pending_schedule": {
    "schedule_lib_num": 0,
    "schedule_hash": "0000000000000000000000000000000000000000000000000000000000000000",
    "schedule": {
      "version": 0,
      "producers": []
    }
  },
  "activated_protocol_features": null,
  "additional_signatures": []
}

I've confirmed with eosjs 22.1.0, 21.0.4, and 20.0.3 that operation works as expected. (20.0.3 does not use get_block_header_state for transact() though, instead just always calling get_block)

Resolves #39

[ericpassmore]

Note:start
group: PROTOCOL
category: HTTP
summary: Restore get_block_header_state endpoint for usage both pre and post Savanna activation.
Note:end