Add full support for subscription

[seriyps]

Implemented according to https://spec.graphql.org/October2021/#sec-Subscription

The idea is that the callback module that backs the subscription field exports 2 functions: normal execute/4 but also subscribe/3.

subscribe/3 should initiate the subscription (it returns some abstract subscription identifier)
execute/4 is implemented almost as any other execute/4 but the "Source Object" (2nd argument) of this function will be the 2-tuple: forementioned "subscription identifier" and the message received from pub-sub system:

case graphql:execute(Ctx, AST) of
    #{subscription := {Identifier, OpaqueContext}} ->
        receive {my_pubsub, Tag, Message} ->  % in real code it will be `handle_info` gen server callback
           assert_match(Identifier, Tag),
           #{data := ...} = graphql:handle_subscription_event(#{}, Identifier, OpaqueContext, Message),
           ...
        end;
   #{data := ...} -> ...