return contract_abi in Stacks transaction metadata

[rafaelcr]

The Stacks node sends the contract_abi in RPC events whenever it finds a transaction that deployed a smart contract:

    {
      "txid": "0xb55e53065b3e934716eb0f7dcce5138b719ea247459f18f60893e15c8d9a0159",
      "raw_tx": "0x00000... (truncated)",
      "status": "success",
      "tx_index": 49,
      "raw_result": "0x0703",
      "burnchain_op": null,
      "contract_abi": {
        "maps": [
          {
            "key": {
              "tuple": [
                { "name": "name", "type": { "buffer": { "length": 48 } } },
                { "name": "namespace", "type": { "buffer": { "length": 20 } } }
              ]
            },
            "name": "map_claimed_bns_note",
            "value": "bool"
          },
          {
            "key": "uint128",
            "name": "map_claimed_member_note",
            "value": "bool"
          }
        ],
        "epoch": "Epoch24",
        "functions": [

It would be extremely useful to the Metadata API (and other users) if that contract ABI was included as part of the transaction metadata in chainhook events. Something like:

      "transactions": [
        {
          "metadata": {
            "description": "deployed: SP2C2YFP12AJZB4MABJBAJ55XECVS7E4PMMZ89YZR.arkadiko-dao",
            "execution_cost": {
              "read_count": 41,
              "read_length": 366,
              "runtime": 165143000,
              "write_count": 47,
              "write_length": 31680
            },
            "contract_abi": {
        "maps": [
          {
            "key": {
              "tuple": [
                { "name": "name", "type": { "buffer": { "length": 48 } } },
                { "name": "namespace", "type": { "buffer": { "length": 20 } } }
              ]
            },
            "name": "map_claimed_bns_note",
            "value": "bool"
          },
          ...

Implementation idea: Hide this feature behind CONTRACT_ABI flag. Make it opt-in (default FALSE).