tfe_organization_members: add email address field

[kaefferlein]

Description

addresses #1998

The data source tfe_organization_members now includes the email address of a user.

The official API doesn't document the email address in the API response. However, it is actually included. I confirmed this with a local curl, and it's written in the go-tfe client as well.

Previous output of a member

{
  organization_membership_id = "ou-1234"
  user_id                    = "user-someid"
}

New output:

{
  organization_membership_id = "ou-1234"
  user_email                 = "someone@tfe.com"
  user_id                    = "user-someid"
}

Testing plan

No specific testing plan, I did test it manually against a playground TFE setup:

1. Create a main.tf with the following content:

provider "tfe" {
  hostname = "my-org"
  token        = "my-token"
}

data "tfe_organization_members" "my_org" {
  organization = "my_org"
}

output "members" {
  value = data.tfe_organization_members.my_org.members
}

2. Run a terraform plan to check the outputs.
3. Check the user_email field is present.

Changes

The tfe_organization_members data source now contains the user_email for members / members_waiting.