refactor(types,register): introduce IdUrl and related types into submod

- `IdUrl` does basic validation for identity URL. Server could enforce
  additional restrictions on their own need.

- single-label doamins are now rejected by default.

- More tests are added for `IdUrl` validation.

blah-types/Cargo.toml

@@ -16,6 +16,7 @@ serde = { version = "1", features = ["derive"] }
 serde_jcs = "0.1"
 serde_json = "1"
 serde_with = "3.9.0"
+thiserror = "1.0.63"
 url = { version = "2", features = ["serde"] }
 
 [dev-dependencies]