Implement SMI v1alpha2 router

[stefanprodan]

This PR adds support for SMI TrafficSplit v1alpha2.

For service mesh providers that require this version set:

apiVersion: flagger.app/v1beta1
kind: Canary
metadata:
  name: my-canary
spec:
  provider: "smi:v1alpha2"

Ref: #551
Ref: openservicemesh/osm#1081

Note that both v1alpha3 and v1alpha4 are backwards compatible with TrafficSplit v1alpha2. For newer versions the v1alpha2 must be set in the CRD e.g.:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: trafficsplits.split.smi-spec.io
spec:
  group: split.smi-spec.io
  scope: Namespaced
  names:
    kind: TrafficSplit
    listKind: TrafficSplitList
    shortNames:
    - ts
    plural: trafficsplits
    singular: trafficsplit
  versions:
    - name: v1alpha4
      served: true
      storage: true
    - name: v1alpha3
      served: false
      storage: false
    - name: v1alpha2
      served: false
      storage: false