Add a 'Remote XSI' class for running tests agains RTL sim

[CiaranWoodward]

This remote XSI class behaves very similarly to the previously existing XSI class, with the exception of:

1. The 'create' function doesn't actually use the program args provided
2. The sim_args provided are ignored (and also they are xsim args that wouldn't make sense to RTL sim)
3. The XE file will be loaded before the connection is made (so XE is just for symbols)
4. The RTL implementation of XSI can currently only target a single tile

The actual "Remote XSI" RPC implementation is shipped in the internal tools package. The Remote XSI class will only be defined when the internal tools are installed.