public List<Extension> getExtensions() {
      return extensions;
    }
Represents the extension <code>metainfo.xml</code> file.

An extension version is like a stack version but it contains custom services.  Linking an extension
version to the current stack version allows the cluster to install the custom services contained in
the extension version.
ambari-server/src/main/java/org/apache/ambari/server/state/stack/ExtensionMetainfoXml.java