Introduce combinator type.

[jameskentdev]

Summary

This PR is the start of adding support for filtering with OR instead of only AND as mentioned in the docs. The solution is the one proposed in #415, which introduces a new type as follows:

@type combinator :: %Combinator{type: :and | :or, filters: [Filter.t() | combinator()]}

The changes focus on just introducing the new type, and adding support for it inside Flop.Validate.

To get this to work, I introduced a new library - polymorphic_embed. This makes it easier to support filters containing either a Filter or a Combinator type.

[woylie]

Hey @jameskentdev, thanks for working on this. I don't want to add a dependency for this, though. Could you find another way to implement this?

[jameskentdev]

I took a look at some other approaches in jameskentdev#1, I'm still working on finishing it up. Just for arguments sake I did also add the last few changes here to get it all working with this dependency.