Implement NGINX ingress header regex matching

[stefanprodan]

This PR allows specifying a header regex expression in the canary analysis that will translate to canary-by-header-pattern annotation on the canary ingress object.

Example:

apiVersion: flagger.app/v1beta1
kind: Canary
spec:
  ingressRef:
    apiVersion: networking.k8s.io/v1beta1
    kind: Ingress
    name: podinfo
  analysis:
    match:
    - headers:
        x-user:
          regex: ".*insider.*"

Note that the canary-by-header-pattern feature is not yet released but it's scheduled for v0.31.0 kubernetes/ingress-nginx#5114

Fix: #545

[mathetake]

LGTM