[Access] Implement WSBrokerHandler component

[Guitarheroua]

Based on the Draft design of new WebSockets, a new WSBrokerHandler should be implemented as part of the new WebSocket subscription system, building upon the current WSHandler.

The new WSBrokerHandler should be created in router.go, similar to how the WSHandler is implemented. It can use a similar WSHandler approach:

flow-go/engine/access/rest/routes/websocket_handler.go

Lines 250 to 268 in 36f067d

	func NewWSHandler(
	logger zerolog.Logger,
	api state_stream.API,
	subscribeFunc SubscribeHandlerFunc,
	chain flow.Chain,
	stateStreamConfig backend.Config,
	) *WSHandler {
	handler := &WSHandler{
	subscribeFunc: subscribeFunc,
	api: api,
	eventFilterConfig: stateStreamConfig.EventFilterConfig,
	maxStreams: int32(stateStreamConfig.MaxGlobalStreams),
	defaultHeartbeatInterval: stateStreamConfig.HeartbeatInterval,
	activeStreamCount: atomic.NewInt32(0),
	HttpHandler: NewHttpHandler(logger, chain),
	}
	return handler
	}

A new AddPubSubRoute function should be added to router.go to separate the new WebSocket implementation from other REST API functionalities:

flow-go/engine/access/rest/routes/router.go

Lines 63 to 79 in ce74abc

	func (b *RouterBuilder) AddWsRoutes(
	stateStreamApi state_stream.API,
	chain flow.Chain,
	stateStreamConfig backend.Config,
	) *RouterBuilder {
	for _, r := range WSRoutes {
	h := NewWSHandler(b.logger, stateStreamApi, r.Handler, chain, stateStreamConfig)
	b.v1SubRouter.
	Methods(r.Method).
	Path(r.Pattern).
	Name(r.Name).
	Handler(h)
	}
	return b
	}

The main difference in the creation of this handler is that it will use a single route with the Name "ws" and Pattern "/ws", but without a handler function, as specific subscriptions will be managed internally by the new WebSocketBroker.