feat: support api layer and global headers for testcase

examples/api/get.yml

@@ -0,0 +1,22 @@
+name: ""
+request:
+  method: GET
+  url: /get
+  params:
+    foo1: bar1
+    foo2: bar2
+  headers:
+    Postman-Token: ea19464c-ddd4-4724-abe9-5e2b254c2723
+validate:
+  - check: status_code
+    assert: equals
+    expect: 200
+    msg: assert response status code
+  - check: headers."Content-Type"
+    assert: equals
+    expect: application/json; charset=utf-8
+    msg: assert response header Content-Type
+  - check: body.url
+    assert: equals
+    expect: https://postman-echo.com/get?foo1=bar1&foo2=bar2
+    msg: assert response body url