Include total user count in directory group (get/list) response

[Ayushi3333]

For our end users, we want to know the number of users in a directory group before we assign them to teams in our app. Currently, we are having to fetch all the users (using list_users) to obtain the total number which is expensive and makes it easy to go over the rate limit.

We would appreciate if we could get the total user count in a directory group in one or both of the directory group endpoints (list and get).