Add a new API to allow custom client options #106

public AzureAppConfigurationOptions ConfigureClientOptions(Action<ConfigurationClientOptions> options)

using HttpClient client = new HttpClient(
    new HttpClientHandler()
    {
        Proxy = new WebProxy(new Uri("http://example.com"))
    });

builder.AddAzureAppConfiguration(o =>
    o.Connect(...)
     .ConfigureClientOptions(co => co.Transport = new HttpClientTransport(client));
)