actix_multipart::Multipart: replace Rc with Arc

[jb-alvarado]

Hello,
I wanted to ask if it would be possible to change the reference-counting pointer in actix_multipart::Multipart from Rc to Arc.

I would like to use actix_multipart::Multipart in a multithreaded context, but this requires Send. Unfortunately, Rc does not support Send.

Is the overhead for actix manageable? I know that actix is single threaded by default, but there are situations where you want to use a different runtime or need to spawn a task.