Add DASH HA session API design.

[r12f]

This change is made to add HA session API HLD and P4 changes for SAI API generation.

To describe how the HA session API works, the HLD in this change contains:

• The key fundamental components that used to form the HA set topology and their SAI API design.
• How we could implement HA in DASH behavior model and how each components works with each other.
• The life of the packet for inline flow sync.
• The SAI API call sequence of several typical HA related work flows.

The change also contains the P4 code that generates the SAI APIs. And the generated APIs can be previewed in this branch here: https://github.com/r12f/SAI/tree/user/r12f/ha-api/experimental.

[r12f]

Woot! Thanks @mukeshmv and @marian-pritsak for reviewing! I am merging it now.