chore: microgen - add noxfile for the microgenerator

[chalmerlowe]

Adds a noxfile.py to assist with testing the microgenerator and performing tasks such as blacken and lint.

• Follows: PR feat: microgen - adds config file #2284 (should be merged after that PR is merged.)

[leahecole]

Is this noxfile derived from other noxfiles we use? If so, is there anything we need to do to make sure this is set up to be kept up to date by our other automation? If not, I would just ask we open an internal tracking bug to get to that at some point

[chalmerlowe]

Is this noxfile derived from other noxfiles we use? If so, is there anything we need to do to make sure this is set up to be kept up to date by our other automation? If not, I would just ask we open an internal tracking bug to get to that at some point

The file is based on some of the noxfiles we already use.

We don't yet have a plan in place on where to store this microgenerator or how to ensure that it gets run on the GAPIC generated code.

@Linchin has the task to look into much of that. I believe that handling the noxfile is a part of that overall task.

I am tracking some of the requests from this PR series (including this request) to keep them all in place for once we get the alpha fully merged and can start looking at the next steps.