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-core-tests/monitoring/MonitoringTest.cpp

@@ -329,4 +329,4 @@ TEST_F(MonitoringTestSuite, TestHttpClientMetrics)
     ASSERT_STREQ("TcpLatency", GetHttpClientMetricNameByType(HttpClientMetricsType::TcpLatency).c_str());
     ASSERT_STREQ("SslLatency", GetHttpClientMetricNameByType(HttpClientMetricsType::SslLatency).c_str());
     ASSERT_STREQ("Unknown", GetHttpClientMetricNameByType(HttpClientMetricsType::Unknown).c_str());
-}
+}