Custom chain IDs cannot be represented

[StarknetChainId](https://docs.rs/starknet_in_rust/0.3.0/starknet_in_rust/definitions/block_context/enum.StarknetChainId.html) doesn't allow for arbitrary custom chain IDs.

We know of at least one such custom chain Starkware has created -- so it would be great if there were a way of using custom chain IDs (a chain ID is a just a felt value, after all).