Support group approvals

[jfreda]

This PR adds support for Google Group approvers. Google Groups will be listed as approvers first in the document header, followed by individual approvers. When a member of a Google Group approver approves the document, they will be added as an individual approver.

Breaking changes

• The Admin SDK API must be enabled in Google Workspace (this is required to query Google Groups)
• New https://www.googleapis.com/auth/admin.directory.group.readonly scope is required

New (optional) config

google_workspace {
  ...
  // groups_prefix is the prefix to use when searching for Google Groups.
  groups_prefix = "team-"
  ...
}

New APIs

• /groups: searches Google Groups by prefix
  • Method: POST
  • Params: query

Modified APIs

• /approvals/{document_id}
  • Added method: OPTIONS