Users would appreciate the ability to substitute parameters within responses received from a server before forwarding them to the caller. This would allow for dynamic modification, such as altering Time-To-Live (TTL) values, to enhance client software testing.
**Is your feature request related to a problem? Please describe.** I would appreciate being able to substitute parameters in responses from the server before forwarding the revised response to the caller. This would improve the potency of qontract for testing client software in particular, for instance to be able to modify the Time-To-Live (TTL) value a server returns which would then cause the client to send a request to refresh (extend) the TTL far sooner than waiting for the period set in the server's response. As an example (based on https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control), if a server responds with `Cache-Control: public, max-age=604800, immutable` I would love to be able to specify a new value for the max-age parameter that would be applied in-situ. **Describe the solution you'd like** There are various ways this could be specified, for instance a simplistic approach could be something along the lines of: `var response = str.replace("max-a