Add IPC actor bindings to the repo

[aakoshh]

IPC Solidity actors live here: https://github.com/consensus-shipyard/ipc-solidity-actors

The IPC Agent has copies of the generated ABI here: https://github.com/consensus-shipyard/ipc-agent/tree/main/contracts

These are used with abigen! here.

In my experience this doesn't work with Rust Analyzer, we just get {unknown} and no code completion. If we actually generate the Rust artifacts, it is more helpful. Since these contracts are more essential to what we are doing than, say, the SimpleCoin contract I used in examples, I think it's worth adding them to the repo.