[BUG] [csharp-netcore] Memory leak in ApiClient

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• [N/A] Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• [N/A ] What's the actual output vs expected output?
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Description

In csharp-netcore, using RestClient as the "library" option (this is the default), the ApiClient class create an instance of a RestClient for each call to Exec() and ExecAsync(). RestClient implements the IDisposable interface because they create a HttpClient when constructed. The HttpClient also implements IDisposable and must be called to free it's resources.

Both Exec() and ExecAsync() do not directly or indirectly dispose of the RestClient . Each time a HTTP request is made using the auto-generated client, a HttpClient is leaked.

openapi-generator version

v6.2.1

OpenAPI declaration file content or url

N/A

Generation Details

N/A

Steps to reproduce

Create a client using the csharp-dotnet generator, leaving the "library" option set to default (i.e. RestClient).
Instantiate the client.
In a loop make 100 requests using the client. Set a break point after the loop
Debug the program.
At the breakpoint, investigate the memory and you will see 100 HttpClient objects in memory.

Related issues/PRs

Could not find a related issue.

Suggest a fix

Reference to mustache file that generates the Client: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache

For C# 8.0 and up, you could add "using" on the same line that the RestClient is created for both Exec and ExecAsync. If you need to support older versions of C#, you could wrap the entire usage of the rest client in

using (RestClient  RestClient client = new RestClient(clientOptions)
                .UseSerializer(() => new CustomJsonCodec(SerializerSettings, configuration)))
      { 
      // The rest of the Exec<T>/ExecAsync<T> method body
      }

Start with modules/openapi-generator/src/main/resources/csharp-netcore/ApiClient.mustache and inspect how Exec() and ExecAsync() create and use RestClient. Reproduce the issue with repeated generated-client requests and observe HttpClient instances in memory. Done means the generated client no longer leaves one HttpClient per request undisposed, while both execution paths continue to work.