Prepare sensitive_url for crates.io (#8223)

Another good candidate for publishing separately from Lighthouse is `sensitive_url` as it's a general utility crate and not related to Ethereum. This PR prepares it to be spun out into its own crate.


  I've made the `full` field on `SensitiveUrl` private and instead provided an explicit getter called `.expose_full()`. It's a bit ugly for the diff but I prefer the explicit nature of the getter.
I've also added some extra tests and doc strings along with feature gating `Serialize` and `Deserialize` implementations behind the `serde` feature.


Co-Authored-By: Mac L <mjladson@pm.me>

common/monitoring_api/src/lib.rs

@@ -195,7 +195,7 @@ impl MonitoringHttpClient {
             endpoint = %self.monitoring_endpoint,
             "Sending metrics to remote endpoint"
         );
-        self.post(self.monitoring_endpoint.full.clone(), &metrics)
+        self.post(self.monitoring_endpoint.expose_full().clone(), &metrics)
             .await
     }
 }