dotnet-svcutil can use default credentials by default

[mconnew]

When creating the HttpClientHandler for fetching the wsdl and any included schema files, HttpClientHandler.Credentials is set to CredentialCache.DefaultCredentials. We shouldn't set this by default, and instead should have a command line parameter to specify using default credentials.

As per documentation, dotnet-svcutil should only be used with wsdl url's that you trust, so this isn't considered a security bug. It is a defense in depth improvement.