Bug fixes and a new feature

• server_certificate_verifier extended to reuse built-in verifier #2064
• Assertion failed when destroying httplib::Client
  #2068
• Spaces incorrectly allowed in header field names #2096
• build(meson): copy MountTest.MultibytesPathName files #2098
• Remove SSLInit #2102
• Add zstd support #2088
• Question the behavior of method read_content_without_length #2109
• Crash when calling std::exit while server running or client requests in flight #2097