feat: support api layer and global headers for testcase

examples/ref_testcase_test.json

@@ -0,0 +1,18 @@
+{
+    "config": {
+        "name": "reference testcase test",
+        "base_url": "https://postman-echo.com",
+        "variables": {
+            "os_platform": "ios"
+        }
+    },
+    "teststeps": [
+        {
+            "name": "run demo_httprunner.json",
+            "testcase": "examples/demo_httprunner.json",
+            "variables": {
+                "os_platform": "$os_platform"
+            }
+        }
+    ]
+}