Expose citation processing logic to callers

[jamesbraza]

As of paper-qa==5.10.2 we expose the citation LLM prompt as a setting in Settings.parsing.citation_prompt/structured_citation_prompt, but then we hardcode the citation processing for MLA parenthetical in-text citation style: https://github.com/Future-House/paper-qa/blob/v5.10.2/paperqa/docs.py#L296-L326

It would be good to expose a hook for citation processing, so users can have references include metadata like source filename.