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/testing-resources/include/aws/testing/mocks/aws/client/MockAWSClient.h

@@ -58,9 +58,10 @@ class AmazonWebServiceRequestMock : public Aws::AmazonWebServiceRequest
     Aws::Client::CompressionAlgorithm GetSelectedCompressionAlgorithm(Aws::Client::RequestCompressionConfig config) const override {
         if (config.useRequestCompression == Aws::Client::UseRequestCompression::ENABLE && m_body) {
             m_body->seekg(0, m_body->end);
-            size_t bodySize = m_body->tellg();
+            std::streampos pos = m_body->tellg();
             m_body->seekg(0, m_body->beg);
-            if (bodySize >= config.requestMinCompressionSizeBytes) {
+
+            if (pos != std::streampos(-1) && static_cast<size_t>(pos) >= config.requestMinCompressionSizeBytes) {
               // todo does hit in dry run?
                 return Aws::Client::CompressionAlgorithm::GZIP;
             }