Add Server-Sent Events Sample

[adeutscher]

Server-Side Events can be used to send updates to a connected client in real time on one connection. This implementation uses the MQTT protocol to share events in such a way that multiple API instances could share events (local testing uses MiniStack to support MQTT over WebSockets).

Changes:

• Add /messages endpoint to submit messages and /messages/event-stream endpoint to listen for messages.
  • Users can only receive messages that they themselves have sent, as the MQTT topic that events are published under is derived from the user's ID.
• OpenAPI/Swagger does not understand handling event streams, so explicitly declared a class that could. This event stream listener attempts to behave like a Swagger class as much as possible, such as throwing SwaggerExceptions on HTTP errors.
• Documented pattern in docs/patterns/server-sent-events.md, including generated code samples in several languages.

Tests Run:

• Successfully sending messages.
• Users successfully receive their own messages.
• Users do not receive messages meant for other users.

[adeutscher]

Everything still works, and testing was run. Can't think of a reason not to merge this.