Feature Request: Adding header elements on the fly

[svbergmann]

There is a feature which can replace certain header elements via the tigerproxy. We want to use this feature, but this is only working if the element is already existing. The code is as follows:

Proxy:
  type: tigerProxy
  tigerProxyConfiguration:
    modifications:
      - condition: "isRequest"
        targetElement: "$.header['X-Flow-Id']"
        replaceWith: "123456789"

Other keys are ommitted for clarity.
This code only works if the header element "X-Flow-Id" is present. Is there a way to just add this header element if it is not present?