Refactor simple-game-server

[Mark Mandel]

https://github.com/googleforgames/agones/blob/main/examples/simple-game-server/main.go is getting really large, and a little unweildy!

Let's take some time to refactor it it into something that's a bit more manageable.

Some suggestions include:

• Use a map of commands rather than a giant switch statement
• Split into multiple files (where it makes sense)
• A lot of the SDK calls, such as setAnnotation(...) have very similar function wrappers - can we generalise this in some way?

agones/examples/simple-game-server/main.go

Lines 605 to 611 in d85a5ae

 func setAnnotation(s *sdk.SDK, key, value string) { 
 	log.Printf("Setting annotation %v=%v", key, value) 
 	err := s.SetAnnotation(key, value) 
 	if err != nil { 
 		log.Fatalf("could not set annotation: %v", err) 
 	} 
 } 

...and anything else that jumps out!