enh: Adding annotation and UrlRewrites property to configure istio routes

[brightsparc]

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What this PR does / why we need it: Enables providing custom configuration to may url prefix to rewrite configuration for istio VirtualService.

In particular it allows creating the VirtualSerivce with a list of prefixes/model-endpoints/v2/model-one that maps to an endpoints /v2/model-1 which might not have the same model name eg:

apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: seldon-predictor
spec:
  gateways:
  -istio-system/seldon-gateway
  hosts:
  - "*"
  http:
  - route:
    - destination:
        host: seldon-predictor.canary.svc.cluster.local
        port:
          number: 8000
    match:
    - uri:
        prefix: /model-endpoints/v2/model-one
    rewrite:
      uri: /v2/model-1

Which issue(s) this PR fixes: Fixes #3470

Special notes for your reviewer: This is an example of SeldonDeployment configuration that includes the new annotations as well as optional urlRewrites dictionary to map specific routes inside the container:

apiVersion: machinelearning.seldon.io/v1
kind: SeldonDeployment
metadata:
  name: example-seldon
  annotations:
    "seldon.io/istio-host": "seldon.io"
    "seldon.io/istio-prefix": "/model-endpoints/v2"
    "seldon.io/istio-rewrite": "/v2"
spec:
  protocol: seldon
  urlRewrites:
    "/model-endpoints/v2/model-one": "/v2/model-1"
    "/model-endpoints/v2/model-two": "/v2/model-2"
  predictors:
  - componentSpecs:
    - spec:
        containers:
        - image: seldonio/mock_classifier:{VERSION}
          name: classifier
    graph:
      name: classifier
      type: MODEL
    name: model
    replicas: 1

[seldondev]

Hi @brightsparc. Thanks for your PR.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ukclivecox]

We plan to close this unless you have time to review further @brightsparc

[brightsparc]

We plan to close this unless you have time to review further @brightsparc

I have worked around this further up the stack, fine to close.

[ukclivecox]

Thanks @brightsparc