Pass a string with binary data of the certificate used to
connect to the HTTPS proxy. The format must be "P12" on Secure Transport or
Schannel. The format must be "P12" or "PEM" on OpenSSL. The string "P12" or
"PEM" must be specified with #CURLOPT_PROXY_SSLCERTTYPE.
This option is an alternative to #CURLOPT_PROXY_SSLCERT which instead
expects a file name as input.