Add a modifier for changing HTTP headers

Consider adding a modifier that allows modifying HTTP headers. This could be pretty much the same as `$replace`, but for HTTP headers.

There are several applications for such a header:
1. Emulating a cookie (can help with cookie notices). On the other hand, there's already a scriptlet for that.
2. Change user agent in some requests.
3. Change referrer.
4. Set own policies, etc.

You can do A LOT with this so it must be a trusted-level modifier.

We need to see how many upvotes this feature request gets.

Discussion:
https://github.com/AdguardTeam/AdguardForWindows/issues/1720