Implement SMI v1alpha3 router

[stefanprodan]

This PR adds support for SMI TrafficSplit v1alpha3.

For service mesh providers that require this version set:

apiVersion: flagger.app/v1beta1
kind: Canary
metadata:
  name: my-canary
spec:
  provider: "smi:v1alpha3"

Note that for SMI compatible service mesh solutions like Open Service Mesh, Consul Connect or Nginx Service Mesh,
Prometheus MetricTemplates can be used to implement the request success rate and request duration checks.

Fix: #551

[codecov-commenter]

Codecov Report

Merging #896 (593f450) into main (fcac992) will increase coverage by 0.05%.
The diff coverage is 59.79%.

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
+ Coverage   56.58%   56.64%   +0.05%     
==========================================
  Files          68       69       +1     
  Lines        5565     5662      +97     
==========================================
+ Hits         3149     3207      +58     
- Misses       1946     1977      +31     
- Partials      470      478       +8     

Impacted Files	Coverage Δ
pkg/router/factory.go	0.00% <0.00%> (ø)
pkg/router/smi_v1alpha3.go	65.16% <65.16%> (ø)

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