Add passive topic listen to Socket

[michaeljguarino]

Absinthe has a unique protocol on top of pheonix wss to support GraphQl subscriptions, in that each subscription document sent creates a shadow channel that the client then has to manually subscribe to. I couldn't find any existing function to support it using PhoenixClient.Socket so I implemented this, and it seems functional from my testing.

The process to create a working graphql subscription publishing to the current process becomes:

with {:ok, %{"subscriptionId" => id}} <- Channel.push(absinthe, "doc", %{query: query, variables: variables}),
      do: Socket.listen(@socket_name, id, self())