Create variant of getResourcesOfType that restricts query to specific site/contexts

ContentHostingService should implement a new method as follows, to address issues described in SAK-14165:

/**

• Retrieve a collection of ContentResource objects of a particular resource-type in a set of contexts
  *
• @param resourceType select resources where CONTENT_RESOURCE.RESOURCE_TYPE_ID equals resourceType
• @param context select resources where CONTENT_RESOURCE.CONTEXT in [context,...]
• @return collection of ContentResource
  */
  public Collection<ContentResource> getContextResourcesOfType(String resourceType, Set<String> contextIds);