Add service orchestration capabilities

[kylinsoong]

What's the real requested?

Add API Call orchestration capabilities that can orchestrate multiple API calls across multiple nginx instances?

Simply use a orchestration configuration yaml file to define the orchestration pipeline:

orchestration:
- step: Call SVC 1
   method: GET
   call: http://service.1
   response: ${svc_1_response}
- step: Call SVC 2
   method: POST
   call: http://service.2
   body: ${svc_1_response}
   response: ${svc_2_response}
- step: Call SVC 3
   method: POST
   call: http://service.3
   body: ${svc_2_response}

then there should be a orchestration engine to execute the API call pipeline. GCP has do similar things as [1]

Why request this?

The feature will related nginx-ui with application & data plane persona, not only for nginx ops.

[1] https://cloud.google.com/workflows