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.lock

@@ -644,7 +644,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
- "sysinfo",
+ "system_monitor",
 ]
 
 [[package]]
@@ -1093,6 +1093,13 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "system_monitor"
+version = "0.1.0"
+dependencies = [
+ "sysinfo",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.20.0"