Add ApiClient.mustache for http client library (OpenAPITools#8859)

* add api client mustache for http client

* clean up api client template for restsharp

* test c# httpclient in appveyor

* remove apiclient.cs

* regenerate apiclient.cs

* set library for http client

* remove Libraries

* use libraries folder

appveyor.yml

@@ -43,6 +43,8 @@ build_script:
   - dotnet build samples\server\petstore\aspnetcore-3.0\Org.OpenAPITools.sln
   # build C# aspnetcore 2.2 server
   - dotnet build samples\server\petstore\aspnetcore\Org.OpenAPITools.sln
+  # build C# API client (httpclient)
+  - dotnet build samples\client\petstore\csharp-netcore\OpenAPIClient-httpclient\Org.OpenAPITools.sln
   # build C# API client (netcore)
   - dotnet build samples\client\petstore\csharp-netcore\OpenAPIClient\Org.OpenAPITools.sln
   - dotnet build samples\client\petstore\csharp-netcore\OpenAPIClientCore\Org.OpenAPITools.sln
@@ -64,6 +66,8 @@ build_script:
   # run the locally installed openapi-generator-gradle-plugin
   - gradle -b modules\openapi-generator-gradle-plugin\samples\local-spec\build.gradle buildGoSdk --stacktrace
 test_script:
+  # test c# API client (httpclient)
+  - dotnet test samples\client\petstore\csharp-netcore\OpenAPIClient-httpclient\src\Org.OpenAPITools.Test\Org.OpenAPITools.Test.csproj
   # test c# API client (netcore)
   - dotnet test samples\client\petstore\csharp-netcore\OpenAPIClientCore\src\Org.OpenAPITools.Test\Org.OpenAPITools.Test.csproj
   - dotnet test samples\client\petstore\csharp-netcore\OpenAPIClient\src\Org.OpenAPITools.Test\Org.OpenAPITools.Test.csproj

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java

@@ -583,8 +583,11 @@ public void processOpts() {
             additionalProperties.put("useRestSharp", true);
             needsCustomHttpMethod = true;
         } else if (HTTPCLIENT.equals(getLibrary())) {
+            setLibrary(HTTPCLIENT);
             additionalProperties.put("useHttpClient", true);
             needsUriBuilder = true;
+        } else {
+            throw new RuntimeException("Invalid HTTP library " + getLibrary() + ". Only restsharp, httpclient are supported.");
         }
 
         String framework = (String) additionalProperties.getOrDefault(CodegenConstants.DOTNET_FRAMEWORK, defaultFramework.name);
@@ -607,7 +610,6 @@ public void processOpts() {
         }
 
         strategy.configureAdditionalProperties(additionalProperties);
-
         setTargetFrameworkNuget(strategy.getNugetFrameworkIdentifier());
         setTargetFramework(strategy.name);
         setTestTargetFramework(strategy.testTargetFramework);