onboard abstract method
Onboard method is invoked when an atsign is activated for the first time from a client app.
Step 1. Perform cram auth
Step 2. Generate pkam, encryption keypairs and apkam symmetric key
Step 3. Update pkam public key to secondary
Step 4. Perform pkam auth
Step 5. Update encryption public key to server and delete cram secret from server
Set atOnboardingRequest.publicKeyId
if pkam auth mode is PkamAuthMode.sim
Implementation
Future<AtOnboardingResponse> onboard(
AtOnboardingRequest atOnboardingRequest, String cramSecret);