Surface the allowable storage layouts for a given machine

[webteam-app]

Bug originally filed by caleb-ellis at https://bugs.launchpad.net/bugs/1950395

Related bug: https://bugs.launchpad.net/maas/+bug/1949455

We now surface "custom" as an option when applying the storage layout for a machine, however it's only possible for machines that have provided the custom configuration at commissioning time. We should surface a list of allowable storage layouts on the machine websocket endpoint so we can filter the list of storage layout options in the dropdown ahead of time.