Add support for dynamic metadata in http ext authz

Description: We have a use case that requires emitting specific information from the external authz service into the logs, and this can be achieved using dynamic metadata. In the grpc ext_authz, dynamic metadata can be included directly as part of CheckResponse. However, in the http ext_authz, the only method to pass this information is by sending it as response headers, which are then emitted via the dynamic_metadata_from_headers.

It would be great if the dynamic_metadata_from_headers field can be exposed

proposal: In the current implementation, allowed_upstream_headers is already implemented as headersToBackend with support for the exact match type only. We can extend the functionality to include dynamic_metadata_from_headers to support this.

headersToMetadata seems like a suitable option for dynamic_metadata_from_headers. However, I welcome any feedback and suggestions

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: SecurityPolicy
metadata:
  name: ext-auth-example
spec:
  extAuth:
    failOpen: false
    http:
      backendRefs:
      - group: ""
        kind: Service
        name: http-ext-auth
        port: 9002
      headersToMetadata:
      - x-user-id
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: backend