feat: support Apollo HTTP callback protocol for subscriptions

[dariuszkuc]

Is your feature request related to a problem? Please describe.
GraphQL subscriptions enables clients to receive real-time data. While WebSocket based implementations are currently the most popular options, they come with a big drawback - for each subscription you need to maintain persistent connection.

Describe the solution you'd like
Apollo router supports HTTP callback mechanism that allows subgraphs to post their updates without the need to maintain persistent connection. By using callbacks we can scale our subgraphs more easily.

Describe alternatives you've considered
N/A

Additional context
HTTP callback protocol documentation