Mage attachment rest endpoints return a count

Currently, there is no easy way to get the attachment count of a page (see https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/XWikiRESTfulAPI#HAttachmentsresources)

• no explicit count is ever returned
• with the default parameters, the full list of attachments is returned, so it's possible to have the total count from the length of the attachments array, at the price of a possibly very large response body
• a number parameter exists, but only truncate the size of the returned attachments array, without providing the total count

Note: this issue is about attachments count, but the same is true for other resource kinds (e.g., xobjects).