PendingMessagePage
One page of SecureMessageClient.pendingMessages (docs/outbound-message-lifecycle.md): messages in ascending PendingMessage.sequence order.
nextAfterSequence is the sequence of the last message if more pending messages existed when the page was read; pass it as afterSequence for the next page. null means the enumeration reached the end; messages sent later have higher sequence numbers, so to see them, continue after the last sequence you received.