Validation for user's name and email

Email validation is still primitive but checks if the email could theoretically exist (name@domain.com). Separation of Subscriber type into a module as well as actions that cane be performed on it. Updates to the subscribe functionality to align with new test cases that have been made.

tests/health_check.rs

@@ -144,4 +144,33 @@ async fn subscribe_returns_400_for_missing_form_data() {
             "The API did not fail with 400 Bad Request when the payload was {}.", error_message
         );
     }
+}
+
+#[tokio::test]
+async fn subscribe_returns_a_400_when_fields_are_present_but_empty() {
+    // Arrange
+    let app = spawn_app().await;
+    let client = reqwest::Client::new();
+    let test_cases = vec![
+        ("name=&email=ursula_le_guin%40gmail.com", "empty name"),
+        ("name=Ursula&email=", "empty email"),
+        ("name=Ursula&email=definitely-not-an-email", "invalid email"),
+    ];
+
+    for (body, description) in test_cases {
+        // Act
+        let response = client
+            .post(&format!("{}/subscriptions", &app.address))
+            .header("Content-Type", "application/x-www-form-urlencoded")
+            .body(body)
+            .send()
+            .await
+            .expect("Failed to execute request.");
+
+        // Assert
+        assert_eq!(400,
+            response.status().as_u16(),
+            "The API did not return a 400 Bad Request when the payload was {}.", description
+        );
+    }
 }