New classes `WeightedAggregationFactory` and `NonWeightedAggregationFactory` replace the previous `AggregationProcessFactory` in the API

This change makes explicit difference between ways to create `tff.templates.AggreagationProcess` objects with input type signatures of its `next` function `<state, value>` and `<state, value, weight>`.

Also updates `ClippingFactory` and `ZeroingFactory` to implement both newly created classes.

`MeanFactory` now only implements `WeightedAggregationFactory` class.

PiperOrigin-RevId: 341988708

Adds functions for building useful compositions of aggregators. Also removes ZeroingClippingFactory because it was determined that composing Zeroing and Clipping is superior (since it allows independent estimators for the norms).

PiperOrigin-RevId: 341517003