Enable config to be used as context manager

[eriknw]

Small enhancement to config to allow it to be used with with ...:.

CC @rossbar, weren't you interested in using context managers? For example (and CC @Schefflera-Arboricola):

with nx.config.backends.parallel(n_jobs=8):
    ...

Our config uses a simple stack to handle contexts and is unapologetically global. It does not even attempt to work well in multithreaded code via thread locals or async code via context variables (and trying to support both is non-trivial, so applications should roll their own solutions as necessary). #7345 actually adds a note to the config docs about this:

This is a global configuration. Use with caution when using from multiple threads.