Add invalidate client library cache support

[cliffano]

From time to time, developers reported that a dispatcher cache hadn't been flushed successfully and they had to resort to invalidating client library cache by hitting /libs/granite/ui/content/dumplibs.rebuild.html?invalidate=true manually on AEM Publish instance(s).

This needs to be added to the API spec and included in the generated client, in order to allow the integration at the upper layers.