Implements RFC 8470 “Using Early Data in HTTP”

[arturobernalg]

• Retries once on 425 Too Early.
• Optionally retries idempotent requests on 429/503, honoring Retry-After (delta-seconds or HTTP-date).
• Ensures retries only for idempotent methods and repeatable entities/producers.
• Signals TLS layer to disable early data on retry (http.client.tls.early_data.disable).

[ok2c]

@arturobernalg Do I understand it correctly this change-set implements a subset of RFC 8470, namely the retry part, and does not implement early data requests at all?