submitChallenge
Submits a challenge that is requested by the server. Sometimes, when sending a message, the server may rate limit the sender (signald has ProofRequiredError), requiring the user to either submit a push challenge (on Android, this is via Firebase Cloud Message) or a reCAPTCHA. This is indicated by the property org.inthewaves.kotlinsignald.clientprotocol.v1.structures.JsonSendMessageResult.proofRequiredFailure, which corresponds to ProofRequiredException in the Signal-Android code.