api, metadata: use map type for Container.Extensions

To allow for updating extensions without collisions, we have moved to
using a map type that can be explicitly selected via the field path for
updates. This ensures that multiple parties can operate on their
extensions without stepping on each other's toes or incurring an
inordinate number of round trips.

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

api/next.pb.txt

@@ -190,7 +190,7 @@ file {
       number: 10
       label: LABEL_REPEATED
       type: TYPE_MESSAGE
-      type_name: ".google.protobuf.Any"
+      type_name: ".containerd.services.containers.v1.Container.ExtensionsEntry"
       options {
         65001: 0
       }
@@ -234,6 +234,27 @@ file {
         json_name: "options"
       }
     }
+    nested_type {
+      name: "ExtensionsEntry"
+      field {
+        name: "key"
+        number: 1
+        label: LABEL_OPTIONAL
+        type: TYPE_STRING
+        json_name: "key"
+      }
+      field {
+        name: "value"
+        number: 2
+        label: LABEL_OPTIONAL
+        type: TYPE_MESSAGE
+        type_name: ".google.protobuf.Any"
+        json_name: "value"
+      }
+      options {
+        map_entry: true
+      }
+    }
   }
   message_type {
     name: "GetContainerRequest"