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