Allow to define union availability on types

[flobernd]

For situations like in e.g. #2826 we have to include/exclude certain types from union type aliases.

To support this situation, we should introduce a new annotation union_availability which can by places on types. The syntax should copy the availability syntax.

On schema.json level, there should be a new key union_availabilites which contains the defined availabilities. The individual entries should use the Availability type - just like for the regular availabilities dictionary.

Open for discussion:
Should we support the same mechanic for deprecation?

cc @pquentin @swallez