Introducing boolean combinators without deps.

[jameskentdev]

Summary

As mentioned in woylie#596, the goal is to attempt to do this without introducing additional dependencies. I thought of 3 ways to do this, and I wasn't entirely sure which one to go with, so initially I tried them all.

1. Introduce a custom Ecto.Type and just cast this inside the main changesets. However, I ran into an issue with a circular dependency between Flop.Combinator requiring this custom type, and this custom type also requiring Flop.Combinator for its changeset.
2. Introduce an additional embedded schema, as a union of the two:

   embedded_schema do
     field :field, ExistingAtom
     field :op, Ecto.Enum, default: :==, values: @operators
     field :value, Any
   
     field :type, values: @combinator_types
     field :filters, __MODULE__
   end

   The problem with this is that it resulted in a quirky internal API I didn't like, e.g.:

   %Flop.FilterOrCombinator{
     field: nil,
     op: nil,
     value: nil,
     type: :and,
     filters: [
       %Flop.FilterOrCombinator{...},
       %Flop.FilterOrCombinator{...}
     ]
   }

3. Sort of a lightweight approach at polymorphic_embed just focusing on our use-case, this is the one in this PR.