C# Client for Net Core generate port exaustion

Description

I moved from Net Framework 4.7 to Net Core 3.1 and 5 and I have a critical issue.

The chsarp-netcore client uses RestSharp and is not stable with Net Core actually. See this issue.
It seems to be a net core breaking change they won't patch in time and actually this is impacting on RestSharp.

It silently open at every call a new socket, creating a new HttpWebRequest each time.
This is a behavior totally different in NetFramework where it keep the same.
You can reproduce it easily generating a simple client and using it in both verisons.
Test multiple calls and watch the connection in Wireshark, I can further add images or a report if you prefer.

While NetFramework honors the keep-alive header (supported also by the server) in Net Core it doesnt.

• In NetFramework it do the handshake and TLS connection just 1 time, and it consumes just one socket keeping working on it with the next calls.
• In Net Core not, it open a new socket each times.

This is critical because you cannot understand it when moving your client to net core until you have a feedback from the server closing the connections to all.

Swagger-codegen version

swagger-codegen-cli-2.4.8.jar with csharp client.

Suggest a fix/enhancement

Create a new client that support HttpClient for the net core part as OpenApi made.

Start by examining the C# netcore client generation path in swagger-codegen-cli-2.4.8 and the RestSharp usage described in the issue. Reproduce repeated calls under .NET Core while observing sockets, then compare the behavior with .NET Framework. Done means the generated client reuses connections instead of opening a new socket for each call.