Option to remove HTTP headers in proxy mode

[fbs]

In our setup we have user -> karma use authorization, but karma -> alertmanager through a service mesh (istio). When proxy is enabled the headers from the user are copied and the request is seen as coming from a user instead of a service, and breaks.

Karma already has an option to add Headers. Can that be extended to remove headers too?

A new config option seems straight forward.

alertmanager:
  interval: 30s
  servers:
    - name: am1
      uri: http://127.0.0.1:9093
      proxy: true
      remove_headers:
        - my-header

Or does it make more sense to add a null value to the current headers option? e.g.

alertmanager:
  interval: 30s
  servers:
    - name: am1
      uri: http://127.0.0.1:9093
      proxy: true
      headers:
        my-hdr: 123
        hdr-to-remove: ~