refactor(system_monitor): move metrics collection and alerting to dedicated modules

* node_notifier doesn't need sysinfo anymore.
* implement AlertFormatter in node_notifier and its own file.
* implement SystemMonitor, ResourceThreshold, ThresholdViolation, and Severity in system_monitor.
* implement parametrized tests for SystemMonitor.

node_notifier/Cargo.toml

@@ -8,5 +8,5 @@ dotenvy = "0.15.7"
 reqwest = { version = "0.12.22", features = ["blocking", "json"] }
 serde = "1.0.219"
 serde_json = "1.0.140"
-sysinfo = "0.35.2"
 discord_client = { path = "../discord_client" }
+system_monitor = { path = "../system_monitor" }