Tcp round trip time plugin

[Zadamsa]

Summary

Add a new process plugin tcpRtt for IPFIXProbe to compute TCP handshake round-trip time (RTT).

Details

This MR introduces a plugin tcpRtt that tracks TCP SYN and SYN-ACK timestamps and computes the handshake RTT. The implementation includes:

• Implementation of plugin in src/plugins/process/tcpRtt
• IPFIX and UniRec export support
• Registration of the plugin in CMake and RPM specs
• Functional test with a sample PCAP

The RTT is computed only when both SYN and SYN-ACK timestamps are available; otherwise, a dummy value is used.

✅ Checks

• [x] CI pipeline passes (build, tests, rpm build)
• [x] New code follows clang-tidy rules
• [x] The change is clearly related to a specific feature (TCP RTT accounting)
• [x] The MR is rebased on the latest master
• [x] No unrelated changes are included
• [x] Functional test added
• [X] Documentation (e.g. Doxygen) is updated if needed
• [ ] Relevant documentation was updated (if needed)