Ash.Union support

[matt-beanland]

Ash.Union is a wrapper structure with a type and value. ash_outstanding should natively support Ash.Union.

There are lots of permutations possible, and these could involve the DSL, but the simplest behaviour is probably to require actual to be Ash.Union when expected is Ash.Union, not require type to match, and call outstanding on the values, wrapping a non-nil value result back in Ash.Union of the expected type.

Ash.Union https://hexdocs.pm/ash/Ash.Type.Union.html has two forms: the default :type_and_value, and :map_with_tag.

[matt-beanland]

Have done outstanding implementation for Ash.Union :type_and_value