Add optional process.file_activities to link pid with files

[xiangchen96]

No description provided.

[ethhart]

I know capa isn't used in your implementation, but am aware of some of the linkages and see some activity from you guys on the capa repo, so wanted to make you aware that this change breaks the cape model defined in the current version of capa https://github.com/mandiant/capa/blob/master/capa/features/extractors/cape/models.py#L301

Either the results shouldn't contain the empty dict by default (two different models now that have to be accounted for if the file_activites option is enabled which will just be implemented through Optional qualifiers) or (ideally) the capa model needs to be updated. I apologize that I am unable to open a PR.