add support for config functions (on subscriptions) in hydrate/2

[bsanyi]

Environment

• Elixir version (elixir -v): 1.14.5
• Absinthe version (mix deps | grep absinthe): 1.7.4
• Client Framework and version (Relay, Apollo, etc): Graphiql, Absinthe.run

Expected behavior

When using an SDL schema, please support adding a config function to a subscription.

    type RootSubscriptionType {
      subs(id: ID!): String
    }

    def hydrate(%Absinthe.Blueprint{}, _) do
      %{
        subscription: [%{subs: [config: &SomeModule.conf_my_subscription/2]}]
      }
    end

    defmodule SomeModule do
      def conf_my_subscription(%{id: id}, _) do
        {:ok, topic: "topic:#{id}"}
      end
    end

I expect that the subscription { subs(id: "hello") } runs the conf_my_subscription function at the time of
submitting the subscription request, and subscribes the client to the topic "topic:hello".

Actual behavior

Currently the hydration above is considered invalid. There's no way to add a config function to a subscription when using SDL schemas.

Relevant Schema/Middleware Code