[FEATURE REQ] Consider Adding EnvironmentKeyCredential to SCM

[KrzysztofCwalina]

Library name

System.ClientModel

Please describe the feature.

Consider adding

public class EnvironmentKeyCredential : System.ClientModel.ApiKeyCredential
{
    public EnvironmentKeyCredential(string environmentVariable)    
    : base(Environment.GetEnvironmentVariable(environmentVariable)!)
    {}
}

Today, we very often write:

string key = Environment.GetEnvironmentVariable("OPENAI_KEY")!;
ChatClient client = new("gpt-4.1", new ApiKeyCredential(key));

... and this would allow

ChatClient client = new("gpt-4.1", new EnvironmentKeyCredential("OPENAI_KEY"));