[native] Implement getSignedIdentityKeysBlob(...) for native

Summary:
In this diff we implement getSignedIdentityKeysBlob(...) and pass it to nativeGetClientResponsesSelector...) so we can call it from within the function created by getClientResponsesSelector(...) if we need to handle serverRequestTypes.SIGNED_IDENTITY_KEYS_BLOB.

Depends on D7034

Test Plan: See if ClientClientResponse for serverRequestTypes.SIGNED_IDENTITY_KEYS_BLOB is properly constructed and received by keyserver.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7035