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/sensitive_url/Cargo.toml

@@ -5,6 +5,12 @@ authors = ["Mac L <mjladson@pm.me>"]
 edition = { workspace = true }
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+serde = ["dep:serde"]
+
 [dependencies]
-serde = { workspace = true }
+serde = { workspace = true, optional = true }
 url = { workspace = true }
+
+[dev-dependencies]
+serde_json = { workspace = true }