Changes in TransactionRequest to support foreign procedure invocation (FPI)

[igamigo]

In order to support foreign procedure invocation (FPI), we need to make some changes on the client side. Specifically, we need a way for users to define which accounts and also, optionally, which storage keys the transaction would make use of.

On execution, the client can reach out to the node's account state retrieval endpoint in order to provide the executor with the necessary data. Something to keep in mind here is that this would only work for public accounts, and so we would need to provide a way to inject account state manually to support private accounts.