Config format with multicontract support and directory paths

[bbyalcinkaya]

Input contracts can be specified in 2 ways:

1. Directory path
   • Doesn't support multi-contracts. Expects exactly 1 wasm file in ./output.
   • Wasm file is resolved to <contract_dir>/output/*.wasm
2. Dictionary
   • Supports multi-contracts
   • "name" is optional.

Examples:

{
  "contracts": [
    "../../../deps/mx-sdk-rs/contracts/examples/adder",
    {
      "path": "../../../deps/mx-sdk-rs/contracts/examples/adder"
    },
    {
      "path": "../../../deps/mx-sdk-rs/contracts/examples/multisig",
      "name": "multisig-full"
    }
  ]
}

The first two are identical.

Issue: #128