fix: ignore unknown fields returned from server for REST (#777)

Otherwise the client may throw an exception if the server is newer than the client (i.e. some of the returned messages have new fields).
A tracking bug: #780

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

@@ -207,7 +207,10 @@ class {{ service.name }}RestTransport({{ service.name }}Transport):
  {%- if not method.void %}
 
  # Return the response
- return {{ method.output.ident }}.from_json(response.content)
+ return {{ method.output.ident }}.from_json(
+ response.content,
+ ignore_unknown_fields=True
+ )
  {%- endif %}
  {%- endif %}
  {%- endfor %}