Add support for configuring a proxy

[joshfriend]

var handler = new HttpClientHandler {
  Proxy = new WebProxy("http://localhost:8888", false),
  UseProxy = true,
  ServerCertificateCustomValidationCallback = (message, certificate, chain, errors) => true
};
var httpProvider = new HttpProvider(handler, false, new Serializer());
var client = new GraphServiceClient(authenticationProvider, httpProvider);

DO NOT use this in production as it ignores certificate validation errors and opens you up to MiTM attacks. It's great for debugging though.

[peombwa]

MSAL now provides a mechanism to configure a proxy https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/wiki/httpclient