[keyserver] introduce setSubcriptionToDefault option in changeRole

Summary:
This diff introduces a new optional option in changeRole called setSubcriptionToDefault. When setSubcriptionToDefault is set to true the subscription for the membership row will be the default subscription where home and pushNotifs are set to false

Depends on D12065

Test Plan:
Please see the demo video below where I use this option to have auto joined communities be set in the background tab

Reviewers: ashoat, kamil

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12444