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>

beacon_node/execution_layer/src/engine_api/http.rs

@@ -652,7 +652,7 @@ impl HttpJsonRpc {
 
         let mut request = self
             .client
-            .post(self.url.full.clone())
+            .post(self.url.expose_full().clone())
             .timeout(timeout)
             .header(CONTENT_TYPE, "application/json")
             .json(&body);