[nio] Provide more flexibility for server streaming contexts to send messages

[glbrntt]

Similar to #537, we should allows users to have more flexibility in how they send messages. At the moment they only have:

• sendResponse(message:) -> EventLoopFuture<Void>

Ideally there should be options to:

• provide an optional promise
• choose whether to flush or not
• send multiple messages
• manually flush

This would bring it into alignment to the client with changes made in #538.