`polymorphic_embeds_many` seems unfinished.

[hickscorp]

I've been using this lib for a long time, but just ran across use cases as it's the first time I try polymorphic_embeds_many. It seems to me that there's missing functionality.

Consider a schema having two possible types of embeds in a polymorphic_embeds_many "things" field. How do you:

• Add to the list, while leveraging polymorphism (Ideally using the type field in attrs to use the right module to call changeset/2 on)?
• Remove to the list ...?
• Chain calls... Eg model |> add_thing_to_things(%{...}) |> add_thing_to_things(%{...}) ...?

[mathieuprog]

Could you mention how you perform all of the mentioned operations using embeds_many? Then we can check if any of these is not supported by the polymorphic version from this lib.

I will re-open the issue once I have more evidence of a such lacking functionality compared to embeds_many.