[Geneva Exporter] Add env_properties to Geneva overrides header file (open-telemetry#487)

Author: ThomsonTan

exporters/geneva-trace/include/opentelemetry/exporters/geneva/geneva_fields.h

@@ -75,6 +75,8 @@
 #  define ETW_VALUE_SPAN_START        "SpanStart"             /* ETW for Span Start      */
 #  define ETW_VALUE_SPAN_END          "SpanEnd"               /* ETW for Span Start      */
 
+# define ETW_FIELD_ENV_PROPERTIES     "env_properties"        /* ETW event_properties with JSON string */
+
 #  define ETW_FIELD_LOG_BODY          "body"                  /* Log body   */
 #  define ETW_FIELD_LOG_SEVERITY_TEXT "severityText"          /* Sev text  */
 #  define ETW_FIELD_LOG_SEVERITY_NUM  "severityNumber"        /* Sev num   */