refactor(notif): Use discord_client crate

- Moved code into a new Rust crate `node-notifier`
- Create new crate discord client
- node_notifier use now discord client

discord_client/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "discord_client"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+reqwest = { version = "0.12.22", features = ["blocking", "json"] }
+serde = "1.0.219"
+serde_json = "1.0.140"
+
+[dev-dependencies]
+mockito = "1.7.0"