[lib] Create function for extracing keyserver id from object id

Summary:
issue: https://linear.app/comm/issue/ENG-4682/create-a-function-to-extract-keyserver-id-from-ids
We need a function that will allow us to extract the keyserver id from message/thread/etc id

Test Plan: Ran the provided test suite

Reviewers: michal, kamil, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9225