feat(gdd): add select-multiple GDD type (Issue ebu#37)

[Richardpwe]

Adds a select-multiple GDD type for selecting zero, one, or multiple values from a fixed list of options.

select-multiple uses type: "array" and supports string, integer, and number values through items.type. The available options are defined in items.enum, so each selected array item can be validated against the allowed values.

This intentionally differs slightly from the issue examples, which show enum on the top-level schema. Since the field value is an array, this PR places the enum on items.enum to match JSON Schema semantics.

Closes #37.