feature request: add socket at beginning of footnote text

[schlcht]

following a discussion with @u-fischer here I'd like to ask for a socket that's executed right at the beginning of the footnote text. As you might be aware, microtype patches several commands to add protrusion, \@footnotetext being one of them. It would be great to be able to use hooks instead of patching, but as yet the hooks provided in latex-lab-footnotes don't really offer a feasible solution: microtype's \leftprotrusion command peeks ahead, so it would already stumble over the hook management code itself, and furthermore could also be distracted by other code added to the hook by other packages or the user. So it would be great to replace, as Ulrike suggested in the above discussion, \ignorespaces#1 (or just #1) with \socket_use:n{fntext/peek}{#1}, so that microtype could simply plug into that socket.