SSL: support in-memory CA certs for some backends

SSL: support in-memory CA certs for some backends

- New options CURLOPT_CAINFO_BLOB and CURLOPT_PROXY_CAINFO_BLOB to
  specify in-memory PEM certificates for OpenSSL, Schannel (Windows)
  and Secure Transport (Apple) SSL backends.

Prior to this change PEM certificates could only be imported from a file
and not from memory.

Co-authored-by: moparisthebest@users.noreply.github.com
Ref: https://github.com/curl/curl/pull/4679
Ref: https://github.com/curl/curl/pull/5677
Ref: https://github.com/curl/curl/pull/6109

Closes https://github.com/curl/curl/pull/6662