add generators metadata file

[AliceProxy]

Adds a metadata file to the root of the repo that contains info about all the available generators. The contents are populated using the new Makefile target. You can simply run make or make generate to update the file. It aggregates the metadata of each generator's config file.

I was originally going to use a hash of the directory for each generator and store that in the metadata file so that clients can check if their cached version is out of date or not. I decided against this approach for now since it was difficult to standardize hashes between machines, likely in-part due to things like line endings, metadata, and whatnot. Instead, clients will make use of the version field for now when comparing their locally cached instance against what is in the repo.

As far as downloads go, I wasn't too sure what the best way to deal with them are. You can't easily download a specific directory from a GitHub repo without making heavy use of the github API to walk that directory and download all the files while rebuilding the directory structure locally and the github public API has a limit of 60 requests per hours so that's not a feasible option. For now, clients will just download the individual generators.json file to check if they need to update their caches, then download the .zip of the main branch. We could pre-zip each generator directory and store those .zips in the repo but that feels messy/confusing. We could also create an individual release for each generator whenever it changes and store those download URLs for that generator's release in the generators.json file so clients can easily download only the generator they want.