[lib] add new selector and function to get own user's keyserver device ID

Summary:
we'll use this to check if the logged in user already has a keyserver associated with their account. if they do, we'll need to prompt them to either cancel the attempt to add a new keyserver or OK replacing the old keyserver with the new one

i moved the olm session types to their own file to avoid a circular dependency

Depends on D12805

Test Plan: tested in next diff by scanning a second keyserver's QR code and successfully replacing the first keyserver in device list

Reviewers: ashoat, bartek

Reviewed By: ashoat

Subscribers: kamil, tomek

Differential Revision: https://phab.comm.dev/D12809