Subscriptions: bi-directional subscription & publish streaming.

Adds SubscribeTopicEvents proto API which dynamically subscribes to
pubsub topics based on dapr/proposals#52.

This is a basic gRPC implementation of the API whereby, like
Subscription hot-reloading today, subscribing to a topic will reload
_every_ active subscription for the current daprd. In a future PR,
reloading of Subscriptions will be granular to the specific pubsub
topic.

Stream subscriptions are also only active once daprd declares the
application as both present and ready. Dynamic stream subscriptions
should be active both whether a app is running or not, as well as
whether it is ready or not. This will be addressed in a future PR.

Signed-off-by: joshvanl <[email protected]>

pkg/api/grpc/endpoints.go

@@ -47,6 +47,9 @@ var endpoints = map[string][]string{
 	"publish.v1alpha1": {
 		daprRuntimePrefix + "v1.Dapr/BulkPublishEventAlpha1",
 	},
+	"subscribe.v1": {
+		daprRuntimePrefix + "v1.Dapr/SubscribeTopicEvents",
+	},
 	"bindings.v1": {
 		daprRuntimePrefix + "v1.Dapr/InvokeBinding",
 	},