feat(macro): support embedding abi as literal json

[kariy]

Allow embedding the ABI JSON array directly into the macro call as one of the way of passing the ABI. There's no breaking changes because the old behaviours are still preserved.

Usage example:

abigen!(MyContract, [
  {
  "type": "function",
  "name": "foo",
  "inputs": [],
  "outputs": [
    {
      "type": "core::bool"
    }
  ],
  "state_mutability": "view"
}]);

[glihm]

Thanks for this feature @kariy. 🙏