Add support for union of interfaces

[moltar]

Right now only primitive types are supported as unions.

What would support for interfaces look like?

json2jsii/src/type-generator.ts

Lines 576 to 578 in b6091c8

	function supportedUnionOptionType(type: any): type is string {
	return type && (typeof(type) === 'string' && PRIMITIVE_TYPES.includes(type));
	}

json2jsii/src/type-generator.ts

Lines 350 to 352 in b6091c8

	if (!supportedUnionOptionType(option.type)) {
	return undefined;
	}