Allow an asynchronous service to respond immediately

[kevinbader]

When in the reverse-proxy configuration a route has target http and the response_from option is present, the synchronous response is dropped.

+----------+                    +-------+                       +----------------+
|          +------------------->+       +---------------------->+                |
|  Client  |                    |  RIG  |                       |  Microservice  |
|          +<-------------------+       |  X <------------------+                |
+----------+                    +---+---+                       +-------+--------+
               async response       ^      response                     |
                                    |      is dropped                   |
                                    |                                   |
                                    |                                   +
                                    |
                                    |                               later...
                                    |
                                    |                                   +
                                    |                                   |
                                    |         async response            |
                                    |                                   |
                                    +-----------------------------------+

The idea is to make this more flexible:

• Simply return the response as if response_from was not set
• If the response's status code is 202 Accepted, ignore the response and wait for the result

This way, a microservice can return a cached response or an error immediately:

+----------+                    +-------+                       +----------------+
|          +------------------->+       +---------------------->+                |
|  Client  |                    |  RIG  |                       |  Microservice  |
|          +<-------------------+       +<----------------------+                |
+----------+                    +-------+                       +----------------+
                                               bad request!

We could add this as an option to the current configuration but I think that would make an already complex-looking configuration even weirder. So I propose this as a breaking change to be included in the upcoming 3.0 release.

Implementation notes

Take a look at lib/rig_inbound_gateway/api_proxy/handler/http.ex