[backend] Add Paperclip to automatically generate OpenAPI v2 specifications for the REST API

[TobiasDeBruijn]

Basically as the title says.

The advantage of this would be that an OpenApi V2 specification can be automatically generated from the code. This would be helpful for those implementing client libraries for Torrust. It should be quite trivial to add too, I'd be more than happy to PR this.

Thanks.

[mickvandijke]

We would certainly appreciate a PR to implement Paperclip :)

[TobiasDeBruijn]

Awesome.

I'll first need to port Paperclip to support Actix v4, after that it should be easy for Torrust.

[josecelano]

Is this package the one you would use? We have migrated the backend to Axum and I do not know is it has support for it. And it seems it's not maintained anymore. I'm going to transfer the issue to the backend repo discussion and make it a "feature request" discussion.