Ability to specify zip file for .NET Core Lambdas

[mattbarry]

AWS recommends using the AWS Toolkit for Visual Studio or the .NET Core CLI to create deployment packages for the .NET Core runtime: https://docs.aws.amazon.com/lambda/latest/dg/lambda-dotnet-how-to-create-deployment-package.html

Being able to specify a zip file for the deployment package would facilitate the use of these tools.