Add Linux runtime on a web app slot

[nkuzmincodat]

This PR closes #

The changes in this PR are as follows:

• Add ability to specify Linux FX version on a slot for a web app

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

let slot =
      appSlot {
          name "warm-up"
          runtime Runtime.DotNet80
      }

[JustinWilkinson]

Looks good to me :)