Guard curl connection sharing when CURL_LOCK_DATA_CONNECT is undefined

[plopesc]

Fixes #229.

PHP only defines CURL_LOCK_DATA_CONNECT when its curl extension is built against libcurl 7.57.0 or newer. Since 0.0.72, ApiClient::__construct() uses the constant unconditionally, so on older builds (for example PHP 8.3 with libcurl 7.47 on Ubuntu 16.04, still common on managed hosting) every client instantiation throws Error: Undefined constant "CyberSource\CURL_LOCK_DATA_CONNECT".

This change keeps connection sharing exactly as it is on supported builds and skips it on builds where the constant does not exist:

• __construct(): create the share handle only when CURL_LOCK_DATA_CONNECT is defined.
• callApi(): attach CURLOPT_SHARE only when a share handle exists.

Verified on PHP 8.3.31 / libcurl 8.14 (share handle still created, requests unchanged) and on PHP 8.3.30 / libcurl 7.47 (no fatal, requests succeed without connection sharing).

[plopesc]

Verified the patched client on the affected platform as promised:

• PHP 8.3.30, libcurl 7.47.0 (Ubuntu 16.04), defined('CURL_LOCK_DATA_CONNECT') is false.
• Before this change, new \CyberSource\ApiClient($config, $merchantConfig) threw Error: Undefined constant "CyberSource\CURL_LOCK_DATA_CONNECT" (lib/ApiClient.php:117).
• With this change, the client constructs normally and live requests succeed: two MicroformIntegrationApi::generateCaptureContextWithHttpInfo() calls against apitest.cybersource.com (one per merchant configuration, JWT shared-secret authentication) returned valid capture contexts.
• On PHP 8.3.31 / libcurl 8.14 the share handle is still created and requests are unchanged.