Axum API: scaffolding

[josecelano]

Parent issue: #156

We are going to migrate the web framework from Actix Web to Axum.

After implementing this issue, we will be able to run the API either with the current Actix Web implementation or the new Axum implementation. This is only the basic scaffolding to be able to run the tests for the new implementation. We will add more issues for each API context:

• about
• category
• proxy
• settings
• torrent
• user

The new implementation will be executed only for testing. The production code will continue executing only the Actix Web implementation until the Axum one is fully implemented.