Allow user to load custom runtimes as wheels

[adriangonz]

trafficstars

As an extension from #694, instead of a Python script, we could let the user pass along a Python wheel, containing their runtime as a Python package.

Caveats

• It’s not clear whether a wheel can be installed on-the-fly in a running Python interpreter

[adriangonz]

Given this requires tweaking the environment in-the-fly, it makes sense to group it with #721(and move it to the v1.3.x release).

[adriangonz]

For this one, it's better to include any custom wheels as part of the custom environment (see #721 )