Add 'attachmentName' configuration parameter to the 'attach' mojo of the xwiki-platform-tool-xmldoc-update-plugin

If would be nice if we could specify a different attachment name for the file that is to be attached to a page at build time.

If it happens to have a file that you need to attach to page A with name 'a' and to page B with name 'b', you can not use the same file. You need to either make a copy of the file or rename it once you are done with page A.