build(sdk-platform-java): Add formatter plugin to the root pom of sdk-platform-java. (#12739)

Running mvn fmt:format would result in errors in the root pom, because
the [formatter
plugin](https://github.com/googleapis/sdk-platform-java/blob/febe86f1ea4f0b5e6d8454024ed499adb7dc9328/pom.xml#L31-L34)
was not migrated. Add formatter plugin to the root pom of
sdk-platform-java to resolve the issue.

Author: blakeli0

sdk-platform-java/pom.xml

@@ -29,6 +29,13 @@
   <!-- Do not deploy the aggregator POM -->
   <build>
     <plugins>
+      <plugin>
+        <groupId>com.spotify.fmt</groupId>
+        <artifactId>fmt-maven-plugin</artifactId>
+        <configuration>
+          <skip>true</skip>
+        </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-deploy-plugin</artifactId>