fixes for embedded resources within unions

[jsw800]

These changes fix a few issues that I ran into with unions of embedded resources.

What I'm trying to do is:

• a union
• with constraint storage: :map_with_tag
• The individual types are embedded resources
• Some of these embedded resources' attributes include a source field that remaps them from what's in the DB (the DB versions are camel case because of bad past decisions)
• There are field policies on some of the embedded resource fields since they should not be visible to everyone

I've run into a few issues doing this. I worked with opencode to get to some solutions here. I'm struggling to put together good test reproductions since this is kind of a niche use case and I've mostly been testing it against my own codebase.

The issues themselves are:

• loads of embedded resources inside unions were being bypassed by the def load(unions, [], _, _), do: {:ok, unions} function head
  • the fix for this was to remove that function head and simply default the load variable to [] in the main function head
• the __union_tag__ property was not being properly included in a few code paths that handle updates, resulting in errors about No such input type for action <MyAction> (type is my tag key)
  • the fix was to include __union_tag__ in the constraints in those code paths
• field policies were not being properly applied to the fields in the embedded resources
  • the fix was the changes in read.ex to the cleanup_field_auth function. To be transparent, I understand these changes less well than I do the other two—opencode is mostly responsible for these. I would appreciate a bit of a closer look at this section from someone with a much closer understanding of this module's code.

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[zachdaniel]

Seems reasonable to me 😄

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