feature: authorization identity helper method

crates/atproto-xrpcs/src/authorization.rs

···
 #[derive(Clone)]
 pub struct Authorization(pub Header, pub Claims, pub String, pub bool);
 
+impl Authorization {
+    /// identity returns the optional issuer claim of the authorization structure.
+    pub fn identity(&self) -> Option<&str> {
+        if self.3 {
+            return self.1.jose.issuer.as_deref();
+        }
+        None
+    }
+}
+
 impl<S> OptionalFromRequestParts<S> for Authorization
 where
     S: Send + Sync,