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>

validator_client/beacon_node_fallback/src/lib.rs

@@ -656,7 +656,7 @@ impl<T: SlotClock> BeaconNodeFallback<T> {
         R: Future<Output = Result<O, Err>>,
         Err: Debug,
     {
-        inc_counter_vec(&ENDPOINT_REQUESTS, &[candidate.as_ref()]);
+        inc_counter_vec(&ENDPOINT_REQUESTS, &[candidate.server().redacted()]);
 
         // There exists a race condition where `func` may be called when the candidate is
         // actually not ready. We deem this an acceptable inefficiency.
@@ -668,7 +668,7 @@ impl<T: SlotClock> BeaconNodeFallback<T> {
                     error = ?e,
                     "Request to beacon node failed"
                 );
-                inc_counter_vec(&ENDPOINT_ERRORS, &[candidate.as_ref()]);
+                inc_counter_vec(&ENDPOINT_ERRORS, &[candidate.server().redacted()]);
                 Err((candidate.to_string(), Error::RequestFailed(e)))
             }
         }