Add "HTTPSFuture->addCurlOption()" for raw access to "curl_setopt()"

Summary: Fixes T13533. This is a narrow, fragile API for a particular Kerberos use case on one install.

Test Plan:

• Set a non-scalar key, got an exception.
• Set <"duck", "quack">, got an exception from cURL that the value was invalid.
• Set a bunch of made-up options to arbitrary values, no errors. cURL accepts anything so there's nothing we can do about this.
• Set CURLOPT_NOBODY and saw the request behavior change, demonstrating that the call can produce effects.

Maniphest Tasks: T13533

Differential Revision: https://secure.phabricator.com/D21251