Use channels more consistently for endpoint sequences

[johakoch]

The handling of endpoint sequences is currently quite complicated. And sometimes the test TestEndpoint_Sequence/complex_request/proxy_sequences fails.

We could create producers with output and, if needed, input channels that connect the producers and by which producers pass their "product" to other producers.

endpoint "/**" {
  request "a" {
    url = "..."
  }

  request "b" {
    url = "..."
    json_body = backend_responses.a.json_body.foo
  }

  request "c" {
    url = "..."
    json_body = backend_responses.a.json_body.bar
  }

  request "d" {
    url = "..."
    json_body = {
      b = backend_responses.b.json_body
      c = backend_responses.c.json_body
    }
  }

  response {
    json_body = {
      d = backend_responses.d.json_body
    }
  }
}

could lead to

        +----------+
        | pReq "a" |
        +----------+
         |        |
   c0 +--+        +--+ c1
      |              |
      v              v
+----------+    +----------+
| pReq "b" |    | pReq "c" |
+----------+    +----------+
      |              |
   c2 +--+        +--+ c3
         |        |
         v        v
        +----------+
        | pReq "d" |
        +----------+
             |
          c4 |
             v

with e.g. pReq "a" having no input, but two output channels, and pReq "d" having two input and one output channel.

Then call each producer's Produce() in its own go routine. The producers without input channels will produce first, while the others wait for all their input channels to be filled before producing.

See simplified demo at https://go.dev/play/p/JzxaWhxa7Pj and a more elaborate one at https://go.dev/play/p/Tt1JllFoG4v