Infra: manage cluster access from an external data source

[agaudreault]

The list of permission to the GCP account is managed as code, but the users access is given on personal gmail accounts. Some users might not want to have their personal gmail in the source code publicly available.
A Terraform Data Source can be used to retrieve the list of user gmail to use for permissions.

AC

• Use a low-cost data source to store the list of gmails.
  • Could be managed by a github team, and have a data source that contains the mapping of github username <-> gmail account.
• Update documentation on how to add/remove a user
• Update CODEOWNERS based on the github team / users