feat: track UTM query parameters (backport)

This PR adds support for tracking UTM query parameters under the `utm` checkpoint.
All UTM parameters will be captured except for the `utm_id` which could be PII.

## Related Issues

Fix #137

src/index.js

@@ -177,3 +177,11 @@ const observer = new PerformanceObserver((list) => {
   }
 });
 observer.observe({ type: 'resource', buffered: true });
+
+const usp = new URLSearchParams(window.location.search);
+[...usp.entries()]
+  .filter(([key]) => key.startsWith('utm_'))
+  .filter(([key]) => key !== 'utm_id')
+  .forEach(([key, value]) => {
+    sampleRUM('utm', { source: key, target: value });
+  });