Add support for OCSP Stapling to OpenSSL I/O Handler

[rlebeau]

See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_tlsext_status_cb.html

In a nutshell:

• On the client side, prior to starting a SSL/TLS handshake, call SSL_CTX_set_tlsext_status_cb() to enable a status callback function, and SSL_set_tlsext_status_type(TLSEXT_STATUSTYPE_ocsp) to request OCSP from the server. The callback function can then use SSL_get_tlsext_status_ocsp_resp() and d2i_OCSP_RESPONSE() to access the server's OCSP response data.

• On the server side, call SSL_CTX_set_tlsext_status_cb() to enable a status callback function. The callback function can then use SSL_get_certificate() to access the certificate that is being sent to the client, obtain the desired OCSP response to be sent back as needed, and finally use SSL_set_tlsext_status_ocsp_resp() to include that response data with the certificate.