.NET Core migration

[fabiocav]

Opening this issue to track the work and discussions around the .NET Core/.NET Standard migration.

Work is currently in progress and will move through the various repositories, ultimately moving into the Azure Functions runtime (WebJobs Script): WebJobs SDK -> WebJobs extensions -> WebJobs Script.

[fabiocav]

The following issues will track individual extension migration work, in proposed priority.
They'll be handled in distinct PRs and committed as work completes:

• Extension project ([.NET Core] - Migrate extension project #237)
• HTTP Extension ([.NET Core] - Migrate HTTP extension project #238)
• Event Hubs Extension (Migrate Event Hub Extension to .Net Core #267)
• Service Bus Extension (Migrate ServiceBus Extension to .NET Core azure-webjobs-sdk#1329)
• DocumentDB Extension ([.NET Core] - Migrate DocumentDB extension project #240)
• SendGrid Extension ([.NET Core] - Migrate SendGrid extension project #243)
• Twilio Extension ([.NET Core] - Migrate Twilio extension project #242)
• Mobile Apps Extension ([.NET Core] - Migrate Mobile Apps extension project #241)
• Notification Hubs Extension ([.NET Core] - Migrate NotificationHubs extension project #239)
• API Hub Extension ([.NET Core] - Migrate ApiHub extension project #289)
• Webhooks Extensions (Add WebHooks support to Functions 2.x/.NET Core azure-functions-host#1698)