Support custom MUD system ABI

[k1rill-fedoseev]

Each MUD world has a bunch of "systems" contracts associated with it. Systems act pretty much like proxy implementations, but MUD has its own way of storing them, which will work better with a separate tab on MUD contract specifically.

For example, the tab might look like:

99% of functions are write-only, but in some rare cases we might see read functions too. I suggest to display them under the same tab, and put the buttons Read/Write/Simulate accordingly.

API example:

// /api/v2/mud/world/<address>/systems
{
  "items": [
    {
      "name": "sy.AccessManagement",
      "address": "0x17ffdeff94ed0b80c493a179d4b3b09d6d71f627"
    },
    {
      "name": "sy.BalanceTransfer",
      "address": "0xa274b9a7e743cd8df3c6fd0abd47ed55fc943bc3"
    },
    // ...
  ]
}

// /api/v2/mud/world/<address>/systems/<system_address>
{
  "name": "sy.AccessManagement",
  "abi": [
    {
      "inputs": [
        {
          "name": "resourceId",
          "type": "bytes32"
        },
        {
          "name": "grantee",
          "type": "address"
        }
      ],
      "name": "grantAccess",
      "outputs": [],
      "stateMutability": "nonpayable",
      "type": "function"
    },
    ...
  ]
}