This change is part one of three:

1. Add `secure_sum` intrinsic to TFF.
2. Update transformations to support the `secure_sum` intrinsic.
3. Update canonical form to support the `secure_sum` intrinsic.

Here is an example using the API:

```python
  @computations.federated_computation
  def init_fn():
    """The `init` function for `computation_utils.IterativeProcess`."""
    return intrinsics.federated_value([0, 0], placements.SERVER)

  @computations.tf_computation(tf.int32, tf.int32], tf.int32]])
  def work(client_data, client_input):
    del client_data  # Unused
    del client_input  # Unused
    return [1, 1]

  @computations.federated_computation([
      computation_types.FederatedType([tf.int32, tf.int32], placements.SERVER),
      computation_types.FederatedType(tf.int32, placements.CLIENTS),
  ])
  def next_fn(server_state, client_data):
    """The `next` function for `computation_utils.IterativeProcess`."""
    client_input = intrinsics.federated_broadcast(server_state)
    c3 = intrinsics.federated_zip([client_data, client_input])
    client_updates = intrinsics.federated_map(work, c3)
    federated_update = intrinsics.federated_sum(client_updates[0])
    secure_update = intrinsics.secure_sum(client_updates[1], 8)
    new_server_state = intrinsics.federated_zip([federated_update, secure_update])
    server_output = intrinsics.federated_value([], placements.SERVER)
    return new_server_state, server_output

  ip = computation_utils.IterativeProcess(init_fn, next_fn)
```

PiperOrigin-RevId: 290988239