Allow calling API endpoints not in OpenAPI yet

[ehmicky]

The Netlify OpenAPI specification is missing many endpoints and is often lagging behind. Also, we purposely don't add some endpoints there for features that have not been announced yet.

Because of this, we have refrained from using the js-client in many projects, and prefer to call the Netlify API directly (example). However, the js-client provides with a consistent way to request the Netlify API and parse its response, which we should re-use across projects.

Therefore, we should add a way to call API endpoints that are not in OpenAPI yet.