feat: support api layer and global headers for testcase

examples/api/post.yml

@@ -0,0 +1,30 @@
+name: ""
+request:
+  method: POST
+  url: /post
+  headers:
+    Content-Length: "58"
+    Content-Type: text/plain
+    Postman-Token: $session_token
+  body: This is expected to be sent back as part of response body.
+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.data
+    assert: equals
+    expect: This is expected to be sent back as part of response body.
+    msg: assert response body data
+  - check: body.json
+    assert: equals
+    expect: null
+    msg: assert response body json
+  - check: body.url
+    assert: equals
+    expect: https://postman-echo.com/post
+    msg: assert response body url