tcp-socket-base/bulk-send-application: Implement and consume TCP packet retransmission trace

Related discussion: https://groups.google.com/g/ns-3-users/c/q6WGVRJ-8Cc

This MR adds a trace source + accessor for TCP-retransmitted packets in tcp-socket-base. It is triggered in TcpSocketBase::SendDataPacket(), right after the standard m_txTrace, if the packet is a retransmission.

Furthermore, in bulk-send-application, we connect this TcpSocketBase trace to BulkSendApplication::PacketRetransmitted() which itself launches a continuation m_retransmissionTrace of the same type. Together with the corresponding trace source accessor, this enables users to directly consume TCP retransmissions from any BulkSendApplication that uses TcpSocketBase or derived types.

A quick style check via utils/check-style-clang-format.py-format.py (clang-format v15, ubuntu 20.04) shows no remaining style issues.

NOTE: as discussed in aforementioned google group thread, the retransmission trace is of different origin than the traces used in FlowMonitor/FlowProbe, and thus obtained values for retransmitted data packets/bytes are rather rough estimates.

NOTE2: I will squash the commits once this is otherwise ready to be merged.