Mount read-only

**What are you trying to do?**
Mount a host directory read-only in an instance.

**What's your proposed solution?**
Add a `--read-only` option to the `multipass mount` command.

Start by locating the multipass mount command's option parsing and the implementation that attaches a host directory to an instance. Trace how mount options reach the platform-specific mount logic. Done means --read-only is accepted by multipass mount and the mounted host directory cannot be written from the instance.

Written by the indexing model from the issue text.