Refactor ZFS filesystem ensure arguments

[smklein]

          Not really this PR's fault, but it contributes: the args to this method can be borderline unreadable at call sites, IMO. Really wish we had a nice way to do keyed arguments. Maybe this method is worth having something like

pub struct EnsureFilesystemArgs<'a> {
    pub name: &'a str,
    pub mountpoint: Mountpoint,
    pub zoned: bool,
    pub do_format: bool,
    pub encryption_details: Option<EncryptionDetails>,
    pub size_details: Option<SizeDetails>,
    pub id: Option<DatasetUuid>,
}

to help? That would also give a nice place to attach the doc comments, too. (If we don't do this, the doc comment needs a new addition for id.)

Originally posted by @jgallagher in #7236 (comment)