shared/api/storage/pool/volume: Adds custom volume backup structs
shared/api/storage/pool/volume: Adds custom volume backup structs

Co-authored-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>
Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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