Add email sketch.

src/email_client.rs

@@ -0,0 +1,14 @@
+use crate::domain::SubscriberEmail;
+
+pub struct EmailClient;
+
+impl EmailClient {
+    pub async fn send_email(
+        &self,
+        recipient: SubscriberEmail,
+        subject: &str,
+        content: &str,
+    ) -> Result<(), String> {
+        todo!()
+    }
+}

src/lib.rs

@@ -1,5 +1,6 @@
 pub mod configuration;
 pub mod domain;
+pub mod email_client;
 pub mod routes;
 pub mod startup;
 pub mod telemetry;