Add appname_header option

[ShimmerGlass]

This option adds a header in the style of "X-Forwarded-For", but
containing the app name that made the request.
The information is extracted from the client certificate.

Example configuration:

{
  "service": {
    "name": "socat",
    "port": 8181,
    "connect": {
      "sidecar_service": {
        "proxy": {
          "config": {
            "appname_header": "X-App"
          }
        }
      }
    }
  }
}