Send msg.sender along to fallback handler

[rmeissner]

Currently the fallback handler doesn't have any way to detect what contract triggered it.

A similar approach to what GSN is doing can be used (e.g. appending msg.sender to the data).

The fallback handler then can extract the msg.sender (e.g. https://github.com/OpenZeppelin/openzeppelin-contracts/blob/solc-0.8/contracts/GSN/GSNRecipient.sol#L217)

[rmeissner]

We should add a notice that this functionality should only be relied on in the fallback habdler with contracts that are of version 1.3.0 or newer. It might make sense to add a check for this in the HandlerContext contract