Prior to this commit, the WebFlux and WebMVC infrastructure would only
support subscriptions over the WebSocket and RSocket transports.
This commit adds the `GraphQlSseHandler` implementations for both web
frameworks. This handler will send GraphQL responses as as stream of
Server-Sent Events, over an HTTP response with the "text/event-stream"
content type.
This implementation only supports the "Distinct connections mode" and
will reject all operations other than Subscriptions.
This commit also enhances the `HttpGraphQlTransport` client transport to
support subscriptions over this new protocol.
Closes gh-309
1f7063c9b4