Decouple API resources from internal structures

[josecelano]

The API endpoints return internal objects, for example:

The endpoint to get all categories:

http://127.0.0.1:3000/v1/category

returns the struct Category:

pub struct Category {
    pub category_id: i64,
    pub name: String,
    pub num_torrents: i64,
}

That's only one example.

That has some problems:

• If you change the internal representation, you can break the API unintentionally.
• You cannot change the API resources and the internal objects independently. For example, it will be harder to implement version 2 for the API.
• In the future, the API resource could contain different data.
• It's harder to mock the API responses if those objects have extra logic, ar nested, etcetera.

We could change it progressively or when we implement the API v2.

Subtasks

• [x] https://github.com/torrust/torrust-index/issues/501

[cgbosse]

@da2ce7 and @WarmBeer any comments?