…ntials` (#770)

This includes the following properties:

- `info`: This is the reverse of `from_info` defined on subclasses and useful to
  serialize external account credentials.
- `service_account_email`: This is the corresponding service account email if impersonation is used.
- `is_user`: This is `False` for workload identity pools and `True` for workforce pools (not yet supported).
  This can be mainly determined from the STS audience.

While the properties will primarily facilitate integration with gcloud, they are publicly useful for other contexts.

Updates `google.auth.external_account.Credentials#info` to not delete items in the dictionary while iterating.

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>