Factory code easily supports adding Secure Aggregation components

[thomasp-ms]

Is your feature request related to a problem? Please describe.
Currently, if customers wants to add Secure Aggregation (SA) to their Federated Learning (FL) experiment, they are on their own.

Describe the solution you'd like
We would like a solution "a la shrike", where all the customers have to do is implement a few key functions (named generate_noise() and anonymize_model_weights() in shrike). Then the factory code will appropriately handle applying SA.

Describe alternatives you've considered
Haven't considered alternatives, but open to discussion.

Additional context

• Here is the documentation for the shrike FL API. Check it out for instructions on how things are done with shrike.
• Here is the shrike code for the FL API. Look for the generate_noise() or anonymize_model_weights() functions and how they are used, or look for use_secure_aggregation to locate the SA-related parts.
• The pre-built components to generate noise and anonymize model weights can be found here and there.
• An example pipeline using the 2 components above can be found here, and there is the associated config file.

This issue is only about modifying the factory code to introduce the hooks for SA. Once it is closed, then #91 and #92 can be tackled.