Ensure we can decap double-encapsulated packets - Add to DASH HLD Section 2.3

[KrisNey-MSFT]

The source side is covered in DASH. ST will always have 2 encaps. Reply will have 1 encap. If traffic is cross-region, then ST gwy then another layer of encap. SRC mux is not involved. Reply packet goes to SRC SLB Mux for another layer of encap.

ILB on DST side is not covered, however will be double encapp’d.
1 VNET encap, 1 SLB encap.

This is an open item in DASH on the DST side. We need to model this in DASH and add to HLD here:

https://github.com/sonic-net/DASH/blob/main/documentation/general/dash-sonic-hld.md#23-service-tunnel-st-and-private-link-pl-packet-processing-pipelines