Background Worker template

[Tratcher]

A new template for generic host without web host.

No websdk.

Include a Hello World IHostedService

Include health checks (non-http)?

@glennc 's POC https://github.com/glennc/worker

[Tratcher]

Deployment environments:

• Kubernetes (health checks) https://github.com/aspnet/Extensions/issues/967
• Windows service (+installer) Add support for running Generic Host based applications as a Windows Service extensions#809
• Daemon https://github.com/dotnet/corefx/issues/25384

[Tratcher]

@glennc since this will only be using Extensions should it get an optional net4x TFM? Not sure if our template infrastructure supports that.

[Tratcher]

@glennc moving to preview4 to track template upgrades for #6817 (comment)