feat(http-client): Set User-Agent header dynamically

This commit updates the HTTP client code to set the 'User-Agent' header dynamically based on the SDK version. The User-Agent now follows the format 'FlutterwavePHP/{SDK_VERSION}', where {SDK_VERSION} is the version of the SDK obtained from the EnvVariables class.This change improves the clarity of user-agent information for HTTP requests, helping with tracking and debugging.

Author: Abraham-Flutterwave

src/Config/AbstractConfig.php

@@ -43,7 +43,11 @@ protected function __construct(string $secret_key, string $public_key, string $e
             [
             'base_uri' => EnvVariables::BASE_URL,
             'timeout' => 60,
-            RequestOptions::VERIFY => \Composer\CaBundle\CaBundle::getSystemCaRootBundlePath()
+            'headers' => ['User-Agent' => sprintf(
+                'FlutterwavePHP/%d', EnvVariables::SDK_VERSION
+            )],
+            RequestOptions::VERIFY => 
+            \Composer\CaBundle\CaBundle::getSystemCaRootBundlePath()
             ]
         );