[keyserver][lib][native][web] NewThreadRequest -> NewThinThreadRequest
Summary:
Renaming this type to distinguish from thick threads, which are initialized through Olm session over Tunnelbroker.
Depends on D12485
Test Plan: Flow
Reviewers: kamil, marcin
Reviewed By: kamil
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D12486