rfctr(part): add new decorator to replace four

[scanny]

Summary In preparation for pluggable auto-partitioners, add a new metadata decorator to replace the four existing ones.

Additional Context "Global" metadata items, those applied to all element on all partitioners, are applied using a decorator.

Currently there are four decorators where there only needs to be one. Consolidate those into a single metadata decorator. One or two additional behaviors of the new decorator will allow us to remove decorators from delegating partitioners which is a prerequisite for pluggable auto-partitioners.

[Coniferish]

What are the four decorators this is replacing? I'm only seeing three that are used on the partitioners:

• process_metadata
  • Adds .metadata.regex-metadata when regex_metadata keyword-argument is provided.
  • Updates element.id to a UUID when unique_element_ids argument is provided and True.
• add_metadata_with_filetype
• add_chunking_strategy
  • Chunks the element sequence produced by the partitioner it decorates when a chunking_strategy argument is present in the partitioner call and it names an available chunking strategy.

Also, I'm not seeing the chunk addressed in this decorator. Is that coming in a later PR?