Add core plugins to grails plugin metadata

[jdaugherty]

Issue description

Non-default, feature plugins that are included in grails-core need added to the grails plugin metadata index

[jdaugherty]

The plugin metadata repo is here: https://github.com/apache/grails-plugins-metadata/ - we need to add the various optional plugins in grails-core to this list. i.e. the async plugin, etc. We should NOT add core plugins that included with a default app (i.e. i18n)