Automatically generate OPTIONS handlers for all paths

[morremeyer]

It should be possible to automatically generate OPTIONS handlers for all paths.

This would save a lot of the current code since OPTIONS handlers are written manually currently.

The OPTIONS handler for single resource endpoints could even be rewritten to always return the possible HTTP verbs without checking if the resource exists - the endpoint will always accept all the methods, just the HTTP response will differ depending on the resource existing or not.

This could be built into its own repository and module so that it can easily be used by other gin users, too. However, if that makes it more complex to develop and maintain, it should be part of the backend codebase in its own module.