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/events/content.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+package containerd.events;
+
+import "gogoproto/gogo.proto";
+import "github.com/containerd/containerd/protobuf/plugin/fieldpath.proto";
+
+option go_package = "github.com/containerd/containerd/api/events;events";
+option (containerd.plugin.fieldpath_all) = true;
+
+message ContentDelete {
+	string digest = 1 [(gogoproto.customtype) = "github.com/opencontainers/go-digest.Digest", (gogoproto.nullable) = false];
+}