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.

[Caleb-Ellis]

Blocked until LP bug is fixed, then we can filter the list in ChangeStorageLayout.tsx

[tmerten]

please remove blocked in case the second backend bug helps solving this

[ndv99]

@tmerten The second bug already has both UI and Backend fixed and merged (ability to select "Custom" as a layout option), but the backend fix for allowable layouts appears to still be pending