[ENH] add possibility for custom data attributes in plugin button + custom url redirect for plugin button with modal

2 things. currently when you use a plugin button, you can't define your own data attributes (since tiki 19 you can do it for plugin div but not plugin button). this MR adds this functionality.

second, when you use the plugin button with a click-modal mode, you can't custom where the user is redirected once the modal is submitted. by default it's redirected to the same page. this MR adds the possibility to custom your own redirect url with the data-modal-submit-redirect-url data atttribute for the plugin button.