Enforce getSignedIdentityKeysBlob in socket selectors

Summary: Changes introduced in parent differential allow us to make getSignedIdentityKeysBlob a non-optional argument for socket selectors.

Test Plan: Run flow

Reviewers: atul, kamil, michal

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9533