This PR contains the work done to implement the `AuthMiddleware` middleware, to authenticate the requests sent to the backend service, based on [their specifications](https://www.discogs.com/developers/#page:authentication).
In addition, some documentation has been added/updated and some boilerplate source code has been removed from the project.
Reviewed-on: #3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>
javier