expose useful broker and server methods

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

server/context.go

@@ -14,3 +14,13 @@ func FromContext(ctx context.Context) (Server, bool) {
 func NewContext(ctx context.Context, s Server) context.Context {
 	return context.WithValue(ctx, serverKey{}, s)
 }
+
+// SetServerSubscriberOption returns a function to setup a context with given value
+func SetServerSubscriberOption(k, v interface{}) SubscriberOption {
+	return func(o *SubscriberOptions) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, k, v)
+	}
+}