Kafka broker exits when OAuth enabled and certain configuration not specified

The sasl.oauthbearer.jwks.endpoint.retry.backoff.ms and sasl.oauthbearer.jwks.endpoint.retry.backoff.max.ms configuration options were added to the SaslConfig class but their default values were not added to KafkaConfig. As a result, when the OAuth validation feature is enabled in the broker and those two configuration values aren't explicitly provided by the user, the broker exits.

The fix is to define them in the KafkaConfig class.