Use the official Torrust Tracker REST API client crate after publishing it

[josecelano]

Relates to: torrust/torrust-tracker#689
Depends on: torrust/torrust-tracker#689

The Index uses the Torrust Tracker REST API. It uses a custom client for the API. However we are in the process of releasing an official crate. The package has been extracted:

https://github.com/torrust/torrust-tracker/tree/develop/packages/rest-tracker-api-client

but the crate is not published yet.

We should start using that package once it's published.

NOTE: this implementation is better than the one we have in the Tracker. We have to apply the same improvements to the client in the tracker before releasing it.