feature request: add header only if not already set

[gilbsgilbs]

I'm in a case where I'd like the header directive to set the header only if it wasn't already set by an upstream server. Think it as a default value for headers. This would be useful for e.g. setting Cache-Control to no-store by default, except if the upstream server defines a specific Cache-Control value. I don't think it is possible yet (at least with the Caddyfile adapter).

The syntax could be inspired from the syntax used to append/delete headers:

#      ↓
header ?Cache-Control "no-store" {
  defer
}

or alternatively using an explicit parameter (but that wouldn't make much sense when used in combination with +/- IMHO):

header Cache-Control "no-store" {
  if_not_set
  defer
}

Would you accept such contribution? Or is it out of scope?