Ecto 3.12: Add Query.ByExpr structs next to the old Query.QueryExpr struct matching

[sriedel]

Addresses #197

To get mongodb_ecto working with ecto 3.12 I've done the naive thing and added Ecto.Query.ByExpr structs next to the places where Ecto.Query.QueryExpr was present.

After the manually updating ecto to 3.12.5, the existing unit tests are working.

However I would need guidance on how to properly set up the tests to work with both struct types, or how to not throw an error when using with an older ecto version that does not define this struct. Nor do I have any knowledge on how matrix testing works in elixir, so pointers on where the configuration would need to be updated would be appreciated as well :).