EXPOSURES: Add support for specifying the condition which can prevent a task from running

[icnocop]

For example, pending #43

new NuGetPushTaskBuilder()
    .WithName("Publish")
    .If("${{ github.event.inputs.publish == 'true' }}") // something like this should be added
    .WithSearchPath(@"**\*.nupkg")
    .WithApiKey("${{ secrets.NUGET_API_KEY }}")
    .WithDestination("https://api.nuget.org/v3/index.json")
    .Build()

should generate the following YAML:

- name: Publish
   if: ${{ github.event.inputs.publish == 'true' }}
   run: dotnet nuget push "**\*.nupkg" --api-key ${{ secrets.NUGET_API_KEY }} --source "https://api.nuget.org/v3/index.json"

I'm not sure if it should be If, WithIf, or maybe something else.