Define a reusable minsize property in the cloud API schema.
The minsize type is either a uint64 or a string with data units.
Internally, this creates a union (json.RawMessage) type called Minsize
and optional uses of the property are of type *Minsize. We define an
internal decodeMinsize() function to convert *Minsize to uint64. This
is similar to the decodeSize() function in the blueprint, but made
specifically to handle *Minsize.
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