[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