Add git info and Python package info providers

[nicholasjng]

They are classes with minimal state and a no-arguments __call__() operator, giving a dictionary of context values as required.

Tests both providers in a lean unit test each.

[nicholasjng]

It might make sense to move these semantically grouped context values down a notch, and use a top-level key for it:

So that, when e.g. using GitEnvironmentInfo(), the context goes from this (currently)

r = nnbench.BenchmarkRunner()
res = r.run(..., context=(GitEnvironmentInfo(),))

res["context"]
# {"commit": "123456789abcdef", ...}

to this:

res = r.run(..., context=(GitEnvironmentInfo(),))

res["context"]
# prints {"git": {"commit": "123456789abcdef", ...}}

This is possible within the current typing, since the value type is Any.