SSH extra options or default x11 forwarding for multipass launched shells

**What are you trying to do?**
I would like to use gui apps from my multipass vm via x11 through multipass shell or the multipass gui.

**What's your proposed solution?**
Allow extra SSH options to be passed when starting a shell through multipass (needs `-X` to be added). Alternately just always enable forwarding even if the user isn't going to use it.

**Additional context**
The docs here: https://multipass.run/docs/set-up-a-graphical-interface#heading--x11-on-linux say that it is unfortunate that multipass can't accept extra args for ssh so we need to trust a different ssh key and run ssh ourselves. While this isn't a lot of overhead its enough that I'm just never going to run gui apps from multipass.

Start with the X11 setup documentation linked in the issue and trace how Multipass shell and GUI launches establish SSH sessions. Determine whether the project should accept extra SSH options such as -X or enable forwarding by default; done means GUI applications work through the supported launch path without manually running SSH.

Written by the indexing model from the issue text.