feat: Add environment credentials feature tracking to User-Agent

- Add CREDENTIALS_ENV_VARS enum to UserAgentFeature with metric ID 'g'
- Track environment credentials usage in AWSClient::AttemptOneRequest
- Check for AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY presence
- Implements env-credentials SEP specification for business metrics
- Add unit test for environment credentials feature tracking

Resolves environment credentials tracking for User-Agent 2.1 business metrics.

Author: kai-ion

tests/aws-cpp-sdk-monitoring-unit-tests/CloudWatchUnitTest.cpp

@@ -107,7 +107,7 @@ TEST_F(CloudWatchTest, TestUserAgentCompressionTracking) {
   metricDatum.SetTimestamp(Aws::Utils::DateTime::Now());
 
   // Create large metadata to exceed compression threshold
-  std::string largeValue(20000, 'A'); // 20KB value to exceed compression threshold
+  Aws::String largeValue(20000, 'A'); // 20KB value to exceed compression threshold
   Dimension dimension;
   dimension.SetName("LargeDimension");
   dimension.SetValue(largeValue);