[keyserver] add the addViewerAsGhost option to createThread

Summary:
This implements a addViewerAsGhost option to createThread. We use this in the context of farcaster bot to allow commbot to create threads without being added as a member.

Depends on D13811

Test Plan: Tested later in the stack. Confirmed that community threads and sidebar threads created by commbot did not have commbot visible in the members list.

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13819