Improving latency with @defer and @stream directives

[jangko]

Although this is not yet made it's way into the specification, they are very interesting experimental feature that can be added
our execution engine.

This feature requires substantial amount of changes in the execution engine because of:

• requires priority analyzer
• async fields execution
• split response builder
• the response stream mechanism to deliver the result should be agnostic about the transport layer

see also https://graphql.org/blog/2020-12-08-improving-latency-with-defer-and-stream-directives