Overhaul core Tracker: refactor statistics module

[josecelano]

Overhaul core Tracker: refactor statistics module.

Statistics have been split into three parts:

• HTTP stats
• UDP stats
• API stats: the API uses both HTTP metrics and UDP metrics.

I have created a temporary directory (src/packages/) with the new packages. We need to create crates for those packages and move them to the src/packages/ dir.

I didn't want to do that in this PR.

$ tree src/packages/
src/packages/
├── http_tracker_core
│   ├── mod.rs
│   └── statistics
│       ├── event
│       │   ├── handler.rs
│       │   ├── listener.rs
│       │   ├── mod.rs
│       │   └── sender.rs
│       ├── keeper.rs
│       ├── metrics.rs
│       ├── mod.rs
│       ├── repository.rs
│       ├── services.rs
│       └── setup.rs
├── mod.rs
├── tracker_api_core
│   ├── mod.rs
│   └── statistics
│       ├── metrics.rs
│       ├── mod.rs
│       └── services.rs
└── udp_tracker_core
    ├── mod.rs
    └── statistics
        ├── event
        │   ├── handler.rs
        │   ├── listener.rs
        │   ├── mod.rs
        │   └── sender.rs
        ├── keeper.rs
        ├── metrics.rs
        ├── mod.rs
        ├── repository.rs
        ├── services.rs
        └── setup.rs

9 directories, 27 files

The app layers are a little bit different from what I described initially in the issue:

Initial design:

                   Main Torrust Tracker
                              |
              Axum HTTP tracker server (`packages\axum-http-tracker`. This hasn't been extracted yet)
                              |
                 HTTP tracker protocol (`packages\http-protocol`)
                              |
                          Core tracker (`packages\tracker-core`)

Final design:

                   Main Torrust Tracker
                              |
              Axum HTTP tracker server (`packages\axum-http-tracker`. This hasn't been extracted yet)
                              |
                    HTTP tracker core  (`src\packages\http_tracker_core`)
                              |
                 HTTP tracker protocol (`packages\http-protocol`)
                              |
                          Core tracker (`packages\tracker-core`)

I din't want to put stats in the packages\http-protocol because they are no part of the official HTTP tracker protocols. And I didn't want to put it in the axum server because it's code that can be re-used in other server implementations (using other frameworks).

[josecelano]

ACK fd8b57a