Implement ChangePassword

This requests support for exercising the changePassword API using the Couchbase Go SDK. It doesn't appear that this is currently supported. We have the need to change a user's own password and are currently using the "settings/rbac/users" API via https://github.com/couchbase/gocb/blob/master/cluster_usermgr.go#L470.

This requires the user to have the "full_admin" role, which is more permissive than we'd like. Additional context is available in this forum post:

https://forums.couchbase.com/t/role-required-to-change-users-own-password-via-the-settings-rbac-users-api/32558 

We're open to other approaches around constructing the changePassword request using the Couchbase Go SDK if any are available.

Thanks!