Fix type annotation for `request` to indicate it can also be a dictionary

[busunkim96]

https://github.com/googleapis/python-secret-manager/blob/dee9bd0ee381528476e8f6f1755b713238b1f1be/google/cloud/secretmanager_v1beta1/services/secret_manager_service/client.py#L277-L285

    def list_secrets(
        self,
        request: service.ListSecretsRequest = None,
        *,
        parent: str = None,
        retry: retries.Retry = gapic_v1.method.DEFAULT,
        timeout: float = None,
        metadata: Sequence[Tuple[str, str]] = (),
    ) -> pagers.ListSecretsPager:

[busunkim96]

Fixed in #966