Customize Payload on `CONNECTION_INIT`

[abn]

Originally raised in #166 by @serializingme

Another issue I had was the need to have some extra data on the CONNECTION_INIT payload. I modified the connection_init_request in client\subscription.py:138 to add extra content to the returned dictionary:

return {
    "type": GraphQLSubscriptionEventType.CONNECTION_INIT.value,
    "payload": {**extra_data, "headers": {**self.request.headers}},
}