feature: Implement Serialized Entity Pattern (#2150)

* Add files:
- App.java
- Country.java
- CountryDao.java
- CountrySchemaSql.java
- CountryTest.java
- README.md
- serialize-entity.urm.puml
- pom.xml (inherit from parent pom.xml)
Update files:
- pom.xml (parent pom.xml, add 1 module called serialized-entity)

* Fix duplicated error in pom.xml

* Update:
- pom.xml (module servant, changed by error, now fixed)
- pom.xml (module serialized-entity)

* Update:
- pom.xml (changed, artifactId)

* Update:
- pom.xml

Add:
AppTest.java

* Resolved changes required by the reviewer

Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>

serialized-entity/src/test/java/com/iluwatar/serializedentity/AppTest.java

@@ -0,0 +1,23 @@
+package com.iluwatar.serializedentity;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+/**
+ * Tests that Serialized Entity example runs without errors.
+ */
+class AppTest {
+
+    /**
+     * Issue: Add at least one assertion to this test case.
+     *
+     * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+     * throws an exception.
+     */
+
+    @Test
+    void shouldExecuteSerializedEntityWithoutException() {
+        assertDoesNotThrow(() -> App.main(new String[]{}));
+    }
+}