Add DohUrl option for DNS-over-HTTPS support

[ChrisJr404]

Summary

Adds a DohUrl option that exposes libcurl's CURLOPT_DOH_URL, letting a session resolve hostnames through a DNS-over-HTTPS server.

cpr::Session session;
session.SetOption(cpr::DohUrl{"https://dns.google/dns-query"});

Closes #1256.

Details

• New include/cpr/doh_url.h defines DohUrl, following the existing single string option pattern (same shape as Interface, built on StringHolder).
• Registered the header in include/cpr/cpr.h.
• Session::SetDohUrl(const DohUrl&) wraps curl_easy_setopt(..., CURLOPT_DOH_URL, ...), with a matching SetOption overload so it works through the variadic option mechanism. An empty DohUrl clears the option (passes nullptr), matching how SetInterface handles the empty case.
• CURLOPT_DOH_URL has been available since libcurl 7.62, which is below the versions cpr already requires, so no version guard is needed.

Tests

Added DohUrlTests in test/session_tests.cpp covering SetDohUrl, SetOption, and clearing the option. The test server does not run a DoH resolver, so the requests target an IP literal; libcurl only consults the DoH server when it has to resolve a name, so the option is applied and accepted without affecting the request. This mirrors how other curl-option wrappers such as LocalPort are tested.

Verification

Built on Linux with -DCPR_BUILD_TESTS=ON and ran the full suite via ctest: 100% passed (32/32). Changed files were run through the project's clang-format.

[COM8]

@ChrisJr404 thanks for contributing!

For the tests could you add some kind of mich DoH server to to ensure the enabled option actually does something in the backend?