GH-3451. Add a JMH benchmark for variants

Initial impl.

Author: steveloughran

parquet-benchmarks/pom.xml

@@ -52,6 +52,11 @@
       <artifactId>parquet-common</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>org.apache.parquet</groupId>
+      <artifactId>parquet-variant</artifactId>
+      <version>${project.version}</version>
+    </dependency>
     <dependency>
       <groupId>org.apache.hadoop</groupId>
       <artifactId>hadoop-client</artifactId>