Extend the interface of `model_delta_optimizer_process` to accept the new `tff.templates.MeasuredProcess` for broadcast and aggregate functions.

Changes to `model_delta_optimizer_process`:
- Wrap the previous `StatefulFn` arguments in `tff.templates.MeasuredProcess`
  and only use `MeasuredProcess` internally.
- Change the metrics out put to be a namespace separated metrics concatenation:
  `train`, `broadcast` and `aggregation`.

Changes to dependents on `model_delta_optimizer_process`:
-  Change `bulid_federated_averaging_process` and `build_federated_sgd_process`
   to only output the `train` metrics to retain current behavior.
-  Update clipping mean to zip its state on return. Future CLs will convert
   this to a `MeasuredProcess`.
-  Remove default specification of broadcast in robust_federated_aggregation
   reserach.

PiperOrigin-RevId: 317305758