Add spotless plugin (google#2537)

* Adds the `spotless` plugin

* Excludes `Java17` files

Excludes:
- Java17RecordTest.java
- Java17RecordReflectionTest.java

* Fixes formatting in `pom.xml`

pom.xml

@@ -143,6 +143,49 @@
           </execution>
         </executions>
       </plugin>
+      <!-- Spotless plugin: keeps the code formatted following the google-java-styleguide -->
+      <plugin>
+        <groupId>com.diffplug.spotless</groupId>
+        <artifactId>spotless-maven-plugin</artifactId>
+        <version>2.40.0</version>
+        <executions>
+          <execution>
+            <goals>
+              <goal>check</goal>
+            </goals>
+          </execution>
+        </executions>
+        <configuration>
+          <formats>
+            <format>
+              <includes>
+                <include>*.md</include>
+                <include>.gitignore</include>
+              </includes>
+              <trimTrailingWhitespace/>
+              <endWithNewline/>
+              <indent>
+                <spaces>true</spaces>
+                <spacesPerTab>4</spacesPerTab>
+              </indent>
+            </format>
+          </formats>
+          <java>
+            <excludes>
+              <exclude>src/test/java/com/google/gson/functional/Java17RecordTest.java</exclude>
+              <exclude>src/test/java/com/google/gson/native_test/Java17RecordReflectionTest.java</exclude>
+            </excludes>
+            <googleJavaFormat>
+              <style>GOOGLE</style>
+              <reflowLongStrings>true</reflowLongStrings>
+              <formatJavadoc>true</formatJavadoc>
+            </googleJavaFormat>
+            <importOrder/>
+            <removeUnusedImports/>
+            <formatAnnotations/>     <!-- Puts type annotations immediately before types. -->.
+          </java>
+        </configuration>
+      </plugin>
     </plugins>
 
     <pluginManagement>