Add Kotlin value classes sample.

Closes #670

mongodb/kotlin/src/main/kotlin/example/springdata/mongodb/people/EmailAddress.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.people
+
+/**
+ * @author Mark Paluch
+ */
+@JvmInline
+value class EmailAddress(val address: String) {
+
+}

mongodb/kotlin/src/main/kotlin/example/springdata/mongodb/people/Person.kt

@@ -23,6 +23,16 @@ import org.springframework.data.annotation.PersistenceCreator
  *
  * @author Mark Paluch
  */
-data class Person @PersistenceCreator constructor(@Id val id: String?, val firstname: String? = "Walter", val lastname: String = "") {
-	constructor(firstname: String?, lastname: String) : this(null, firstname, lastname);
+data class Person @PersistenceCreator constructor(
+	@Id val id: String?,
+	val firstname: String? = "Walter",
+	val lastname: String = "",
+	val email: EmailAddress?
+) {
+	constructor(firstname: String?, lastname: String) : this(
+		null,
+		firstname,
+		lastname,
+		null
+	);
 }

mongodb/kotlin/src/main/kotlin/example/springdata/mongodb/people/PersonRepository.kt

@@ -38,4 +38,6 @@ interface PersonRepository : CrudRepository<Person, String> {
 	 * Query method requiring a result. Throws [org.springframework.dao.EmptyResultDataAccessException] if no result is found.
 	 */
 	fun findOneByFirstname(firstname: String): Person
+
+	fun findOneByEmail(emailAddress: EmailAddress): Person
 }

mongodb/kotlin/src/test/kotlin/example/springdata/mongodb/people/RepositoryTests.kt

@@ -68,6 +68,23 @@ class RepositoryTests {
 		assertThat(walter.lastname).isEqualTo("White")
 	}
 
+	@Test
+	fun `should find one person by email`() {
+
+		repository.save(
+			Person(
+				null,
+				"Walter",
+				"White",
+				EmailAddress("heisenberg@gmail.com")
+			)
+		)
+
+		val walter = repository.findOneByEmail(EmailAddress("heisenberg@gmail.com"))
+
+		assertThat(walter.email?.address).isEqualTo("heisenberg@gmail.com")
+	}
+
 	@Test
 	fun `should return null if no person found`() {