support arity 3 cast functions

[SteffenDE]

I'm trying to implement ordering a has_many association following the documentation (https://hexdocs.pm/ecto/3.10.1/Ecto.Changeset.html#cast_assoc/3-sorting-and-deleting-from-many-collections). As this lead to some problems that were fixed by elixir-ecto/ecto#4178 (not released yet), I switched to using the current ecto main branch and modified my code according to the new documentation, adding a cast function with arity three, that receives the index as third parameter. This works fine, but not when used in a phoenix form.
When using it together with Phoenix.Component's inputs_for/1, it fails with an error like this:

** (FunctionClauseError) no function clause matching in Phoenix.HTML.FormData.Ecto.Changeset.to_changeset/4

     The following arguments were given to Phoenix.HTML.FormData.Ecto.Changeset.to_changeset/4:
     
         # 1
         %Comment{__meta__: #Ecto.Schema.Metadata<:built, "comments">, id: nil, body: "data1", position: nil}
     
         # 2
         nil
     
         # 3
         Comment
     
         # 4
         &Comment.changeset_with_position/3
     
     Attempted function clauses (showing 4 out of 4):
     
         defp to_changeset(%Ecto.Changeset{} = changeset, parent_action, _module, _cast)
         defp to_changeset(%{} = data, parent_action, _module, cast) when is_function(cast, 2)
         defp to_changeset(%{} = data, parent_action, _module, {module, func, arguments} = mfa) when is_atom(module) and is_atom(func) and is_list(arguments)
         defp to_changeset(%{} = data, parent_action, _module, nil)

So here's a PR that tries to add support for arity three cast functions. I'm not completely sure if I implemented this right, but it seems to work with the added test and also in my Phoenix app.
If I'm not on the right track here, see this as a bug report instead :)

[josevalim]

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