Update TrafficSplit version from v1alphav2 to v1alpha4

[snehachhabria]

SMI has a new version of TrafficSplit, OSM needs to be updated to use this version.

Note: The main feature available with this latest version is that it enables the use of TrafficSpec with TrafficSplit. This upgrade isn't mandatory as we are updating TrafficSpec with #449 so all its features could be used with a TrafficTarget

We can work on this item once we get more clarity from the PMs if it is an immediate requirement for add-on or open source

[michelleN]

• identify there is a conflict between policy and split, then figure out how to program Envoys in the scenario of where policy enforcement prevents split.
• how to bubble up errors and give feedback to user

We'll continue to discus with SMI community how to handle these scenarios in a standardized way.

[ksubrmnn]

Currently blocked on new SMI release with v1alpha3 updates

[michelleN]

Had a few meetings with @ksubrmnn @shashankram @draychev @snehachhabria and this is what came out of those. (Please feel free to comment)

To implement this in a way where routes associated with a traffic target are decoupled from a traffic split, we need to do the following steps in order:

1. Relax logic on how outbound routes get programmed. Outbound routes for clusters should be programmed with wildcard routes.
2. Ensure that routes are programmed via RDS on the server side/inbound clusters(already happening).
3. Any time an SMI traffic split is applied, create an outbound virtual host with weighted cluster information on client with routes pointing to the backends of that traffic split.
4. Apply route information (SMI Traffic Split rules / HTTPRouteGroup) to the virutal host for that traffic split.

[michelleN]

Currently blocked on #2034

[snehachhabria]

closing this issue cause of duplicate issue #2368