GROOVY-3457: Preparing for addition of new StreamingTeamplateEngine

o Enabling selective suppression of specific checkstyle rules for specific code
o Enabling the spock unit testing framework for the groovy-templates subproject
GROOVY-3457: Adding new StreamingTemplateEngine to address >64k issues

As per the issue, the current template engine implementations throw
exceptions for data > 64k. Code ported from a separate github repo at:

https://github.com/mbjarland/groovy-streaming-template-engine