Add support for using an input syntax different than HTML in the conversion filter

The WYSIWYG Editor is using a conversion servlet filter to convert the submitted HTML to the storage wiki syntax. The conversion filter makes the assumption that all WYSIWYG Editor implementations produce HTML. This is not true for the BlockNote integration (XWIKI-23025), where the editor produces a specific JSON syntax. In order to cover this use case we need to make the conversion filter more generic, to allow us to specify an input syntax other than HTML.