Add POM generation for meta-plugins

[jasontedor]

This commit adds a POM generation task for the ZIP distribution for meta-plugins. With this change we also modify MetaPluginBuildPlugin to extend BuildPlugin (because we need POM generation configuration from there), remove standalone and REST test plugins, and manually create an integTest task. To simplify this, we abstract out some shared code from PluginBuildPlugin so that it can be reused in MetaPluginBuildPlugin.

[rjernst]

Hrm, I don't think meta plugins should extend from BuildPlugin. They should not have any code (and build plugin is for a Java code). Meta plugins don't build a jar, they simply zip the zips from other plugins. I think we can just add pom generation directly?

[rjernst]

@jasontedor I opened #28321 with my proposed alternative.