feat: adapt googlepubsub bindings to v3

Changes from v2 to v3:

* The `topic` property for the Channel Binding Object is no more and is now
  provided via the native `address` property of the Channel Object
* Protobuf support now uses the native support by providing the Protobuf
  definition as an inline string of the `schema` property of the Message
  Object and the `schemaFormat` now has valid values for Protobuf 2 and 3

googlepubsub/README.md

@@ -22,7 +22,6 @@ Field Name | Type | Description
 `messageRetentionDuration`|String|Indicates the minimum duration to retain a message after it is published to the topic _(Must be a valid [Duration](https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#google.protobuf.Duration).)_
 `messageStoragePolicy`|[Message Storage Policy Object](#message-storage-policy-object)|Policy constraining the set of Google Cloud Platform regions where messages published to the topic may be stored
 `schemaSettings`|[Schema Settings Object](#schema-settings-object)|Settings for validating messages published against a schema
-`topic`|String|The Google Cloud Pub/Sub Topic name
 
 <a name="message-storage-policy-object"></a>
 
@@ -59,17 +58,17 @@ Field Name | Type | Description
 # ...
 channels:
   topic-avro-schema:
+    address: projects/your-project/topics/topic-avro-schema
     bindings:
       googlepubsub:
-        topic: projects/your-project/topics/topic-avro-schema
         schemaSettings:
           encoding: json
           name: projects/your-project/schemas/message-avro
 # ...
   topic-proto-schema:
+    address: projects/your-project/topics/topic-proto-schema
     bindings:
       googlepubsub:
-        topic: projects/your-project/topics/topic-proto-schema
         messageRetentionDuration: 86400s
         messageStoragePolicy:
           allowedPersistenceRegions:
@@ -153,7 +152,13 @@ components:
             type: protobuf
       contentType: application/octet-stream
       name: MessageProto
-      payload: true
+      schema: |
+        syntax = "proto3";
+
+        message Message {
+          required string message = 1;
+        }
+      schemaFormat: application/vnd.google.protobuf;version=3
 # ...
 ```