Add type alias for event Envelope

Signed-off-by: Derek McGowan <[email protected]>

Author: dmcgowan

api/services/events/v1/doc.go

@@ -16,3 +16,8 @@
 
 // Package events defines the event pushing and subscription service.
 package events
+
+import types "github.com/containerd/containerd/api/types"
+
+// Deprecated: Use [types.Envelope].
+type Envelope = types.Envelope

api/services/ttrpc/events/v1/doc.go

@@ -16,3 +16,8 @@
 
 // Package events defines the ttrpc event service.
 package events
+
+import types "github.com/containerd/containerd/api/types"
+
+// Deprecated: Use [types.Envelope].
+type Envelope = types.Envelope