Introduce subscription hooks

[ardatan]

No description provided.

[dotansimha]

@ardatan can you please do the same for the rest of the events? onConnect, onOperationComplete, onOperation and onDisconnect

[ardatan]

@dotansimha I only added onConnect and onDisconnect because of this interface;
https://github.com/apollographql/apollo-server/blob/2df3df63c3ed746e2a61ca3dbc3414a37d55834c/packages/apollo-server-core/src/types.ts#L34

[dotansimha]

Hmm yeah but there are more, and I think it's important to support them :)

[ardatan]

We don't use options to use these hooks; we call the hooks defined in providers instead.
And apollo configuration doesn't have onOperation and onOperationComplete hooks in its configuration; it will be useless to put them in our code.
But if people wants to use GraphQLModules inside these hooks, they can call injector from the module and use DI with it.
Except these; the PR is ready to merge. It works great with both session scope and building context inside the hook approaches.