Add generators project for current and future generative features

[nickfloyd]

related: #2516

From the reference PR

Given that we are headed down to a more generative SDK (see the discussions #2499 and #2495 and #2517 for more info on that)

IMO the preferred approach would be to have a generators project with namespaced scrips i.e.
Octokit.Generators and the generator above would be something like Octokit.Generators.AsyncPaginationExtension (as an example).

This way, we're establishing the future location for all generators to remove this question later. Also, this will allow us to use CI to build the generators, generate the code, and commit it before the other aspects of the SDK are built.

As a side note, in my perfect world brain, all models and related libs would be hoisted out of this SDK and referenced in as a package so that we could better support versioning and be able to ship SDK core changes separate from SDK model changes.

Let's create a generators project, add the script detailed in the PR, then document how to run and what to expect - we can do a follow-up for CI to implement this more automatedly.