[REQ][csharp-netcore][client] Move `RestClient` to `Configuration`

Created by: Anakael

Is your feature request related to a problem? Please describe.

In csharp generator inner RestClient is saved in Configuration, but in csharp-netcore not and it created for each request. It doesn't allow to configure RestClient, for example, add IAuthenticator from RestSharp and also less perfomant as reusage of RestClient as it was in csharp generator.

Describe the solution you'd like

I'd like to move creating RestClient in Configuration class.

Describe alternatives you've considered

As alternative solution for only authorization IAuthenticator can be added to *Api and can be consumed for new RestSharp when it created.

If moving RestClient in Configuration is ok, I can try to implement it by myself.