Allow to set the msg.sender for SmartContract calls

[juanmardefago]

Currently it's not possible to set the msg.sender while calling contract functions with the ethereum.SmartContract class. This is a feature supported in web3.js for example, which allows to set the msg.sender for contract calls and transactions with the from field of the options object.

This is already useful in some cases for view functions, if the view function has modifiers that only allow certain addresses to access them, but will probably also be really useful if transactions could be sent from subgraphs in the future.

[azf20]

Requested again here #2687 by @dmihal

[sandbochs]

bump, would love to be able to set the from for contract calls

[fordN]

This has been supported for awhile now!