WIP : Update the script for supporting UNION which is a `selector`

This MR is for better supporting extension VK_EXT_descriptor_buffer And following logics are updated accordingly:

• Introduce a new type selector_union in templates/types.h

  • Remove the helper apis that type UNION need
  • Pass the type as a parameter for the generated apis instead
  • Take the Enum type instead of index integer in for loop

• Add attribute selection and selector for types when applicable

• Update the way generating functions when it's a selector union.

Tested: Add new extension VK_EXT_descriptor_buffer and make sure the apis generated as expected.

Signed-off-by: Dawn Han dawnhan@google.com