[Feature] Provide a mechanism to set msg::sender

[alexfertel]

In our in-house unit-testing abstractions (which are based on this PoC), we have tried to find a way to set msg::sender. However, I think there is no way right now, because it is private to the SDK:

pub(crate) static mut SENDER: hostio::CachedOption<Address> = hostio::CachedOption::new(||{
    let mut data = Address::ZERO;
    unsafe {
        hostio::msg_sender(data.as_mut_ptr())
    };
    data.into()
})

Could we make this pub (though global mutable statics need to be treated with care) or is there a way to modify this I am missing?