- `get_auth_list` uses the load to perform ACL lookup but if no `eauth`
  key is present, `__get_acl` immediately returns.
- For token-based authentication, where administrators would also like
  to write a custom eauth ACL module, this update allows `__get_acl` to
  check for such a module, either via the `eauth_acl_module`
  configuration value or fallback to the `eauth` module's `acl`
  function, if defined.

- Includes two tests:
  - Validates that the fallback to checking the static `external_auth` option works as expected.
  - Validates the case where a server is configured to perform external ACL lookups via `eauth_acl_module`, `load["eauth"]` is undefined and `token["eauth"]` _is_ defined.
- Verified via `nox -e 'test-3(coverage=False)' -- tests/pytests/unit/auth/test_auth.py`