Publish as self-contained application and set runtime identifier

[maartenba]

When publishing a web application, make it possible to publish as a self-contained application.
https://twitter.com/vijayankit/status/1377519520810606592

Details:
https://docs.microsoft.com/en-us/dotnet/core/deploying/deploy-with-cli#self-contained-deployment
https://docs.microsoft.com/en-us/dotnet/core/deploying/
https://stackoverflow.com/questions/55206607/deploy-a-self-contained-net-core-application-with-azure-devops-app-service-depl

Related to #373

[maartenba]

Current workarounds:

Alternative 1: add <RuntimeIdentifier>linux-x64</RuntimeIdentifier> to project file and use the Azure plugin's publish options

Alternative 2: use FTP publish, and pick the target runtime identifier here: