Path Tracing Midpoint HLD (#1456)

Path Tracing provides a record of the packet path as a sequence of interface ids. In addition, it provides a record of end-to-end delay, per-hop delay, and load on each egress interface along the packet delivery path.

Path Tracing supports fine grained timestamp. It has been designed for linerate hardware implementation in the base pipeline.

Path Tracing enjoys a very rich ecosystem both from Vendor and Open-Source point of view.

Path Tracing is supported across many HW ASIC including Cisco Silicon One, Cisco Light Speed, Broadcom Jericho, Broadcom Jericho2, Marvell and many others.

Path Tracing has also a very opensource ecosystem that includes the Linux Kernel, FD.io VPP, an open P4 implementation, and support in applications such as Wireshark and TCPDump.

The list routing of platforms that have successfully participated in the Path Tracing interop testing are reported in
draft-filsfils-spring-path-tracing.

This PR provides the HLD to support the PT Midpoint in SONiC.

The document that describes the HLD is available here:
https://github.com/cscarpitta/SONiC/blob/srv6_pt_midpoint_hld/doc/path_tracing/path_tracing_midpoint.md

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>