Added X-Real-Ip usage for logging

[douxxtech]

This PR adds the USE_X_REAL_IP=<true|false> key to the configuration file. If set to true, the server will use the X-Real-Ip header to display the IP in logs, e.g. for reverse proxy setups. There is NO validation of the header, so be sure to retrieve it from a trusted source.

[github-actions]

START + GET /: 155 syscalls
Fastest: 0.000016s (close)
Slowest: 0.837108s (accept)

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     5.15ms    8.63ms 213.71ms   99.52%
    Req/Sec     1.60k   309.92     2.39k    67.38%
  127414 requests in 20.04s, 195.03MB read
  Socket errors: connect 0, read 166, write 0, timeout 0
Requests/sec:   6357.31
Transfer/sec:      9.73MB