[Java][Feign] Expose response headers for feign client

[hugo-ma-alves]

Hello

The objective of this PR is to add the possibility of exposing the response Headers of the request on the feign clients.
This is useful for some situations where the client needs to access information that it is contained on the response headers, for example, headers related with pagination, or resource location after creating a new resource.

I've created a new property called "exposeResponseHeaders", that by default is false, so if it is not defined we keep the same behavior as we have now.
If this property is active the response is wrapped in a new class HttpResponse. This class contains the headers map and the body decoded by jackson to the correct type.

Example of a generated client:

package org.openapitools.client.api;

import org.openapitools.client.ApiClient;
import org.openapitools.client.EncodingUtils;
import org.openapitools.client.model.HttpResponse;

import org.openapitools.client.model.Order;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import feign.*;

public interface StoreApi extends ApiClient.Api {


 /**
  * Delete purchase order by ID
  * For valid response try integer IDs with value < 1000. Anything above 1000 or nonintegers will generate API errors
  * @param orderId ID of the order that needs to be deleted (required)
  * @return HttpResponse<Void>
  */
 @RequestLine("DELETE /store/order/{orderId}")
 @Headers({
   "Accept: application/json",
 })
 HttpResponse<Void> deleteOrder(@Param("orderId") String orderId);

 /**
  * Returns pet inventories by status
  * Returns a map of status codes to quantities
  * @return HttpResponse&lt;Map&lt;String, Integer&gt;&gt;
  */
 @RequestLine("GET /store/inventory")
 @Headers({
   "Accept: application/json",
 })
 HttpResponse<Map<String, Integer>> getInventory();

 /**
  * Find purchase order by ID
  * For valid response try integer IDs with value &lt;&#x3D; 5 or &gt; 10. Other values will generated exceptions
  * @param orderId ID of pet that needs to be fetched (required)
  * @return HttpResponse&lt;Order&gt;
  */
 @RequestLine("GET /store/order/{orderId}")
 @Headers({
   "Accept: application/json",
 })
 HttpResponse<Order> getOrderById(@Param("orderId") Long orderId);

 /**
  * Place an order for a pet
  * 
  * @param order order placed for purchasing the pet (required)
  * @return HttpResponse&lt;Order&gt;
  */
 @RequestLine("POST /store/order")
 @Headers({
   "Content-Type: application/json",
   "Accept: application/json",
 })
 HttpResponse<Order> placeOrder(Order order);
}

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@wing328
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[wing328]

@hugo-ma-alves thanks for the PR. Have you considered the "usual" approach by adding an additional with the "WithHttpInfo" suffix to obtain HTTP headers, status code, etc ?

For usual, I mean other clients (e.g. C#, Ruby, etc) have adopted such approach.

[hugo-ma-alves]

@wing328 Thanks for the tip, I haven't checked the c# generator.

I refactored the code to use the same approach as the C# generator.

After pushing the changes I get this error on Circle CI:
Too long with no output (exceeded 10m0s): context deadline exceeded

Can you force a new build?

[wing328]

Restarted. Let's see how it goes.

[wing328]

LGTM but I've not tested it locally

[hugo-ma-alves]

Thanks
I generated some clients locally. But I still have to generate a couple more, if I find any problem, I'll open a new PR.