[orchagent] Add support for Path Tracing Midpoint

[cscarpitta]

What I did Extended PortsOrch to process the attributes required for Path Tracing.

Why I did it This PR is required for the SRv6 Path Tracing Midpoint feature. HLD: https://github.com/sonic-net/SONiC/pull/1456

How I verified it Added new unit tests to portmgr_ut.cpp, portsorch_ut.cpp and tests/test_port.py.