Add eXact Length parameter when creating magnet URI

[antanilol]

Include the xl (eXact Length) parameter in the magnet URI string inside the function TorrentImpl::createMagnetURI().

Closes #20752.

[thalieht]

This just adds the parameter to "Copy magnet link". qBt won't read it for any purpose (in the current state of the PR).
Is that useful for anything?

Also isn't this just a "promise"? It could be fake so, assuming qBt implements reading it, users will add the magnet link expecting a certain size, forget about it and come back to see a different size (once metadata has been downloaded to get the actual size).

[antanilol]

Is that useful for anything?

Many trackers I know (at least 4) provide only the magnet links and not the torrent file, so, in my opinion this would help the automated systems (like Jackett, prowlarr) to get this information.

Also isn't this just a "promise"? It could be fake

Yes, like the display name parameter, after all. I don't think this simple change could hurt anyone.

Also, I don't see why anyone would fake this information

[Chocobo1]

@antanilol
Thank you!