Add Support for POST APIs with Batch Input in Request Body

[amitksingh1490]

Description:

Currently, batch URLs can be specified using query parameters for upstream APIs that use GET requests. However, for APIs that support batching via POST requests, the input parameters must be sent as a list in the request body in a specific format. The current system does not support this functionality.

Enhance the system to add the capability to handle such batching via POST APIs

[amitksingh1490]

Proposal

if upstream takes body as

[{"key": "id", "value": 1}, {"key": "id", "value": 2}]

type Post {
  id: Int!
  userId: Int!
  title: String!
  body: String!
  user: User
    @http(
      url: "http://jsonplaceholder.typicode.com/users"
      body: [{key: "id", value: "{{.value.userId}}"}]
      batchKey: ["id"]
    )
}

---

if upstream takes body as

[{"id": 1}, {"id": 2}]

type Post {
  id: Int!
  userId: Int!
  title: String!
  body: String!
  user: User
    @http(
      url: "http://jsonplaceholder.typicode.com/users"
      body: [{id: "{{.value.userId}}"}]
      batchKey: ["id"]
    )
}

---

if upstream takes body as

[1,2]

type Post {
  id: Int!
  userId: Int!
  title: String!
  body: String!
  user: User
    @http(
      url: "http://jsonplaceholder.typicode.com/users"
      body: ["{{.value.userId}}"]
      batchKey: ["id"]
    )
}

[meskill]

@amitksingh1490 is the issue resolved by #3140 ?