Add weight/priority system in CompositePostProcessingStrategy

[bertramakers]

Some post-processing strategies might be intended to run before/after others. Right now the only way to ensure this is by registering the strategies in the correct order.

We could improve the developer experience, and flexibility, by adding an option to assign a weight / priority to each registered strategy.

If no weight / priority is assigned, assume a 'neutral' weight / priority by default.
(Eg. when working with weights, use 0 by default and negative / positive values to assign a lower / higher weight)