Implement attachments capture

Summary: This differential adds method that parses logs in search for blob hashes and stores all found hashes to an attachments file.

Test Plan:
The same as for parent differential but this time ensure that attachments file are created especially if you send multimedia messages. Attachments file is
a list of blob hashes separated by newline.

Reviewers: michal, kamil

Reviewed By: kamil

Subscribers: bartek, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10837