Add Maven wrapper to Avatica

A Maven wrapper is a nice idea borrowed from Gradle.

• It makes the project's build fully encapsulated. Users don't have to setup a version of Maven (which could potentially be different than that recommended by Calcite)
• It also allows Calcite to control the Maven dependency (upgrading to newer versions for example) making it transparent to the user.
• Users now will use maven commands as

  ./mvnw clean install
  

  instead

  mvn clean install
  

Presto has done the same – https://github.com/prestodb/presto/blob/master/mvnw

It is also extremely easy to setup. See https://github.com/takari/maven-wrapper .