Create dedicated maven-plugins repository

In the main ISPN repo we have a plugins dir that contains our custom maven plugins used throughout our builds. Currently this dir contains the infinispan-defaults-maven-plugin for extracting code defaults to xsd and the protocol-parser-generator-maven-plugin for generating the hotrod parser.

We should create a new repository under the infinispan org, e.g. github.com/infinispan/maven-plugins, to contain all of our maven plugins that do not depend on ISPN code.

The infinispan-defaults-maven-plugin can't be moved as it depends on infinispan-commons.

https://github.com/infinispan/maven-plugins