Return a list of users for pagerduty_team

[TomTucka]

Addresses #345. Still WIP, Need to figure out tests for this 👍🏼

[stmcallister]

I like where you're going with this! Once you get some tests figured out and implemented I'd be happy to review!

[jbfavre]

I second this PR, because it could help to manage user_notifications using Terraform, without having the need to manage users
It could be the opportunity to enforce users' notifications rules at team level.

However, I also see a possible issue with the logic exposed in #345 : when creating a new layer using the Web UI, and depending on start date, users' will be sorted out by PagerDuty API to keep the schedule order consistent.
So, how to sort team members list to be usable in a layer configuration ?
I don't see any elegant way to automatically reconciliate Terraform and the API. 🤔
WDYT @stmcallister ?

[rmgpinto]

I was looking for something like this to have Pager Duty be our source of truth for alerting teams.
As @jbfavre said, it would be nice to have terraform know all team members, but not necessarily manage them.

[doryer]

Looking forward for this to be merged