[lib] Add getSignedIdentityKeysBlob to getClientResponsesSelector

Summary: Include getSignedIdentityKeysBlob to getClientResponsesSelector

Test Plan: getSignedIdentityKeysBlob will be implemented for native and web in subsequent diffs. For now things continue to work as they did (verified with some basic logging + breakpoints).

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6956