Existing disk modal for instance

[david-crespo]

Used on both instance create and instance edit.

Overall pretty simple, but has to fetch existing disks on mount in order to display the dropdown. That's very easy with React Query. Consider adding a refresh button as a proof on concept, just because it's trivial — refetch(). Selection should be preserved on refresh if the selected disk is still present. As with #405, onSubmit should add the selected disk and associated metadata to an array in form state.

Force attach would be reasonably left out for now. In GCP it means attaching an existing disk in read-only mode even if it's already attached to another instance. RFD 4 says "You can only have a disk attached to one instance at a time."

Figma (instance create)

Figma (instance edit)

Button here:

[david-crespo]

Visual side more or less done, #732 and #733 are the followups.