Ability to opt out of batched execution on batch requests

If a developer wants to preserve context when executing a whole document, there needs to be an option to opt out of batched execution. 

https://github.com/absinthe-graphql/absinthe_plug/blob/master/lib/absinthe/plug.ex#L508
https://elixir-lang.slack.com/archives/C0PR49P4P/p1624455107364500