[REQ] Support client certificates for csharp-netcore

Created by: jornhd

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

The API I'm using authenticates by a client certificate. RestSharp that's used for csharp-netcore, supports this, but as the RestClient is not exposed (like in the old csharp version), it's not possible for me to provide the certificate collection.

Describe the solution you'd like

• Add a new property in the configuration (IReadableConfiguration/Configuration) of type System.Security.Cryptography.X509Certificates.X509CertificateCollection (as the RestSharp property).
• If the new property is not null, set RestClient.ClientCertificates from configuration in ApiClient.Exec and ApiClient.ExecAsync before running the request.

Describe alternatives you've considered

Creating the RestClient in the ApiClient constructor and expose it as a property, like in the csharp version.