The 2.51 microversion adds the 'volume-extended' server
external event to the os-server-external-events API.
This is an admin-only API by default and this event is
currently only used by Cinder as part of extending the
size of an attached volume, and therefore does not
have any CLI or python API binding impacts in the client.
Part of blueprint nova-support-attached-volume-extend
Change-Id: I8293704dbb4f75306fe32d3a0118d5bf42c8457e