Update Traffic Split SMI Spec to v1alpha4 to support A/B Testing

[fsforza]

Please describe the Improvement and/or Feature Request
It would be very nice that Open Service Mesh updates Traffic Split to version v1alpha4 to offer the A/B testing feature
https://github.com/servicemeshinterface/smi-spec/blob/main/apis/traffic-split/v1alpha4/traffic-split.md

Scope (please mark with X where applicable)

• New Functionality [ ]
• Install [ ]
• SMI Traffic Access Policy [ ]
• SMI Traffic Specs Policy [ ]
• SMI Traffic Split Policy [x]
• Permissive Traffic Policy [ ]
• Ingress [ ]
• Egress [ ]
• Envoy Control Plane [ ]
• CLI Tool [ ]
• Metrics [ ]
• Certificate Management [ ]
• Sidecar Injection [ ]
• Logging [ ]
• Debugging [ ]
• Tests [ ]
• CI System [ ]
• Demo [ ]
• Project Release [ ]

Possible use cases
My use case is similar to the one used as A/B testing example in the TrafficSplit v1alpha4 doc:

kind: TrafficSplit
metadata:
  name: ab-test
spec:
  service: website
  matches:
  - kind: HTTPRouteGroup
    name: ab-test
  backends:
  - service: website-v1
    weight: 0
  - service: website-v2
    weight: 100
---
kind: HTTPRouteGroup
metadata:
  name: ab-test
matches:
- name: firefox-users
  headers:
    user-agent: ".*Firefox.*"

I need to route a client call to a specific backend version depending to an http header put into the request.
In this example all the Firefox users, according to the user-agent header, will be routed to the website-v2 Kubernetes service.

Thanks!

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