chore(deps): [#979] add new cargo dep: serde_with

We will add a new enpoint to add tracker keys where some JSON values can
be null:

```console
curl -X POST http://localhost:1212/api/v1/keys?token=MyAccessToken \
     -H "Content-Type: application/json" \
     -d '{
     	   "key": null,
           "seconds_valid": null
         }'
```

We need to set those values to `None` in the Rsut strucut.

Cargo.toml

@@ -66,6 +66,7 @@ serde_bencode = "0"
 serde_bytes = "0"
 serde_json = { version = "1", features = ["preserve_order"] }
 serde_repr = "0"
+serde_with = { version = "3.9.0", features = ["json"] }
 thiserror = "1"
 tokio = { version = "1", features = ["macros", "net", "rt-multi-thread", "signal", "sync"] }
 torrust-tracker-clock = { version = "3.0.0-alpha.12-develop", path = "packages/clock" }