SSE: Add support for SSE handler options (#508)

mcp/sse: add support to provide for options for SSE transport

- Add SSEOptions struct to define SSE handler options.
- Update Signature of NewSSEHandler to accept SSEOptions [braking
change]
- Update unit test
- Update example test

Fixes https://github.com/modelcontextprotocol/go-sdk/issues/507,
https://github.com/modelcontextprotocol/go-sdk/issues/503

examples/server/sse/main.go

@@ -65,6 +65,6 @@ func main() {
 		default:
 			return nil
 		}
-	})
+	}, nil)
 	log.Fatal(http.ListenAndServe(addr, handler))
 }