api/events: split event types from events service

To avoid importing all of grpc when consuming events, the types of
events have been split in to a separate package. This should allow a
reduction in memory usage in cases where a package is consuming events
but not using the gprc service directly.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

api/services/events/v1/doc.go

@@ -0,0 +1,2 @@
+// Package events defines the event pushing and subscription service.
+package events