[lib] Move id conversion functions to lib

Summary: They will be needed in the client code for state check conversion. There's one change: changed the error thrown from ServerError to just Error. On the keyserver side it's caught and rethrown as a ServerError.

Test Plan: yarn flow-all

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7851