PR LGTM :)!

just last question, instead of making external_auth public, we can impl helper fn to set external auth ( like we do for reload filters & consolesettings ).:

@@ -132,11 +132,21 @@ pub struct ConnectionSettings {
     pub max_inflight_count: usize,
     pub auth: Option<HashMap<String, String>>,
     #[serde(skip)]
-    pub external_auth: Option<AuthHandler>,
+    external_auth: Option<AuthHandler>,
     #[serde(default)]
     pub dynamic_filters: bool,
 }

+impl ConnectionSettings {
+    /// Sepcify a function to be used for authentication.
+    ///
+    /// This will take precedence and auth specified in config will
+    /// be ignored.
+    pub fn set_external_auth(&mut self, auth: AuthHandler) {
+        self.external_auth.replace(auth);
+    }
+}
+

though, as you are the one of the users who will be using this, lmk which way seems better, Thank you!

Otherwise, we can have both the derive & make it pub 😅

just last question, instead of making external_auth public, we can impl helper fn to set external auth ( like we do for reload filters & consolesettings ).:

If the method doesn't do anything else besides setting the prop, doesn't seem worth the extra code to me. But if it's some style preference or whatever, feel free to change it up.

If the method doesn't do anything else besides setting the prop, doesn't seem worth the extra code to me

you are right, we can tackle it when we add better way to construct configs without TOML file. 😄