Use @metamask/utils for types

[mcmire]

@metamask/utils now houses most of the types from this package (JsonRpcRequest, JsonRpcResponse, and the like). The only thing that we haven't brought over yet is PendingJsonRpcResponse (this issue is tracked in MetaMask/utils#39). Once this is moved over, we can remove the types from this package and import them from @metamask/utils.

[FrederikBolding]

Done!