Allow to specify server IP address

[zhenyolka]

cpp-httplib uses getaddrinfo to get server IP address to connect to. However sometimes this function might not work. For example when staticially linked with musl binary running on system. that don't have /etc/resolv.conf, android for example. Also plain DNS requests may be spoofed by ISP to block access to sites. Some Russian ISPs do that and with implementing by government National System of Domain Names this problem will grow. Also it affects privacy.
One of solution is to allow projects that use this library specify IP address of server when do HTTP/HTTPS request. So we can resolve IP over DoH/DoT or any other secure way and then pass it to cpp-httplib Client class constructor.
This pull request implements this feature.

[zhenyolka]

@yhirose take a look. This PR for my issue

[yhirose]

@zhenyolka, thank you for the pull request. Could you add unit tests in test/test.cc as I always ask contributors to do so. When it comes to adding more constructors in client, I am not comfortable with them because this feature is rarely used and others users will easily get confused with the parameter ip. I would prefer adding set_ip_address(const std::string &ip) instead. Thanks!

@PixlRainbow, do you have any thought about this PR?

[PixlRainbow]

This is similar to CURL's --resolve example.com:443:198.18.110.10 parameter.
Perhaps instead of set_ip_address(const std::string &ip) to set a single IP, or instead of having a single parameter in the constructor to set ip, you could use set_hostname_addr_map(const std::map<std::string, std::string> addr_map) to provide multiple name resolution so that it can handle the case where one domain returns a HTTP 3xx redirect to a different domain with different IP.

[yhirose]

@PixlRainbow, thanks for the excellent suggestion!
@zhenyolka, set_hostname_addr_map(const std::map<std::string, std::string> addr_map) looks very easy for me to understand what it tries to do instead of adding new constructors with ip. Could you adjust your code accordingly? Thanks a lot!

[yhirose]

@zhenyolka, thanks for the update. Could you add a unit test in test/test.cc to verify your implementation?

[zhenyolka]

@yhirose done. Could you check?

[yhirose]

@zhenyolka, thanks for the excellent contribution!