Feature request: add option for setting X-Real-IP header when using DOH upstream

the use case is:

1. external clients connecting to proxy over DOT
2. proxy connects to upstream using DOH, with `X-Real-IP` header
3. the upstream server can respond correctly depending on source address.

without having the address, the upstream i use (technitium-DNS) can't tell where the clients are since all it can see is the proxy address.
it needs to know the source for proper blocking, and only certain clients are allowed to request some DNS zones and recursion.

this is what the technitium instructions say:
> When using a reverse proxy with the DNS-over-HTTP service, you need to add X-Real-IP header to the proxy request with the IP address of the client to allow the DNS server to know the real IP address of the client originating the request.