[services] Extract multipart handling logic to lib

Summary:
A few things related to multipart parsing will be useful to the backup service:

• general function for handling text fields
• ByteStream alias (changed to use Byte instead of Vec<u8> to avoid unnecessary allocations)
• Also removed a few unnecessary map_err(Box::new) and fixed typos

Depends on D8889

Test Plan: yarn run-integration-tests blob

Reviewers: bartek, patryk, kamil, jon

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8892