Robots.txt generation should allow for path limiters

[HitmanInWis]

On a multi-site implementation it's possible that different sites/domains will share the same sling:resourceType for their pages. However, different sites/domains will want their own robots.txt file if for no other reason than specifying different sitemap URLs.

In order to enable this, it would be great if com.adobe.acs.commons.wcm.impl.RobotsServlet.RobotsServletConfig allowed for a list of paths that, if specified, would limit the config to only be honored for those paths. If the list is empty, the servlet would work for any path with the configured sling:resourceType as it does today, preserving backward compatibility.

I might try to find some time to submit a PR for this.