[BUG][csharp-netcore] No provision to pass cookies to the ApiClient

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
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Description

Their is a provision to pass headers in the configuration using DefaultHeaders. But passing cookie here does not seem to work.

For the below code, a wire capture shows the origin header, but cookie param is not found in the message. It gets saved into DefaultParameters but not consumed.

Org.OpenAPITools.Client.Configuration config = new Org.OpenAPITools.Client.Configuration();
var api = new Org.OpenAPITools.Api.NtpApi(config);
var url = endpoint + "ntp/Policies";
var intersightHeaders = new Dictionary<string, string>()
{
    { "cookie", "Param="+Value },
    { "origin", url }
};
foreach (var h in intersightHeaders)
{
    api.Configuration.DefaultHeaders.Add(h.Key, h.Value);
}

openapi-generator version

latest master

OpenAPI declaration file content or url

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Steps to reproduce

Related issues/PRs

Suggest a fix

We could make a change in the ApiClient, if it comes across a header with Name "Cookie", then it could do the below instead.

diff --git a/modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache b/modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache
index 49b81c72e0..8592f55720 100644
--- a/modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache
@@ -271,6 +271,21 @@ namespace {{packageName}}.Client
             {
                 foreach (var headerParam in configuration.DefaultHeaders)
                 {
+                   if (String.Equals(headerParam.Key, "cookie", StringComparison.OrdinalIgnoreCase))
+                    {
+                        string[] cookieSeparators = new string[] {";"};
+                        string[] EqualSeparator = new string[] {"="};
+                        string[] rawCookieParams = headerParam.Value.Split(cookieSeparators, StringSplitOptions.None);
+                        foreach (string paramset in rawCookieParams)
+                        {
+                            String[] param = paramset.Split(EqualSeparator, StringSplitOptions.None);
+                            if (param.Length == 2)
+                            {
+                                request.AddCookie(param[0], param[1]);
+                            }
+                        }
+                        continue;
+                    }
                     request.AddHeader(headerParam.Key, headerParam.Value);
                 }
             }

Start with modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache and inspect how Configuration.DefaultHeaders are added to requests. Verify that a cookie header is converted into request cookies while other headers, such as origin, continue to be added normally. Done means cookies supplied through DefaultHeaders are present in the generated client's request.