API overhaul: reimplement the current API using Axum about torrust-tracker

[josecelano]

hi @da2ce7 @WarmBeer, two questions:

1. Api versions

Are we going to keep the current API version? This issue only changes Warp with Axum. I will break the API in the next issue: #144

We could keep both. Version v1 could be the current one, and version v2 could be the new one.

I'm working on building the new Axum version without changing the current one. Once we move to Axum, it will be easy to add versions, all of them using Axum.

3. API docs

The API documentation is on the torrust.com site.

If we want to generate the API docs automatically with, for example, the utopia crate we should do it soon. Maybe after finishing the Axum implementation while I'm implementing the new API.

Here you have an example with Auxm: https://github.com/juhaku/utoipa/blob/master/examples/todo-axum/src/main.rs

Since we have to maintain docs for multiple API versions I think it's a good idea to use a tool like that. On the other hand, the API is small and it should not change very often.