fix/feat: nginx canary weight support header

[mumubin]

I have a nginx canary deployment scenario where I hope to support canary weight and header at the same time .
However, after testing, the weight of the canary ingress will not change when there is a header configuration. This PR is to fix this case.

P.S. In the article, it said nginx.ingress.kubernetes.io/canary-by-header and nginx.ingress.kubernetes.io/canary-weight can work together

apiVersion: flagger.app/v1beta1
kind: Canary
metadata:
  name: podinfo
  namespace: flagger-bin
spec:
  provider: nginx
  # deployment reference
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: podinfo
  # ingress reference
  ingressRef:
    apiVersion: networking.k8s.io/v1
    kind: Ingress
    name: podinfo
  # the maximum time in seconds for the canary deployment
  # to make progress before it is rollback (default 600s)
  progressDeadlineSeconds: 6000
  service:
    # ClusterIP port number
    port: 80
    # container port number or name
    targetPort: 9898
  analysis:
    match:
      # curl -H 'X-Canary: insider' http://app.example.com
      - headers:
          x-canary:
            exact: "insider"
    # schedule interval (default 60s)
    interval: 10s
    # max number of failed metric checks before rollback
    threshold: 10
    # max traffic percentage routed to canary
    # percentage (0-100)
    maxWeight: 50
    # canary increment step
    # percentage (0-100)
    stepWeight: 5
    # NGINX Prometheus checks

[codecov-commenter]

Codecov Report

Patch coverage: 75.00% and project coverage change: +0.71% 🎉

Comparison is base (7fc007a) 54.56% compared to head (c0354b1) 55.28%.
Report is 48 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
+ Coverage   54.56%   55.28%   +0.71%     
==========================================
  Files          84       85       +1     
  Lines       10143    10234      +91     
==========================================
+ Hits         5535     5658     +123     
+ Misses       3953     3917      -36     
- Partials      655      659       +4     

Files Changed	Coverage Δ
pkg/router/ingress.go	67.26% <75.00%> (+0.19%)	⬆️

... and 4 files with indirect coverage changes

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[mumubin]

Any questions? We can discuss it.