Create sample(s) for Aspire Dashboard images

[lbussell]

Part of #5128. There should be at least one sample for using the Aspire Dashboard container image as intended. It probably belongs under the kubernetes directory.

[lbussell]

[Triage] There is one sample available in the Aspire samples repo: https://github.com/dotnet/aspire-samples/tree/main/samples/StandaloneDashboard

It needs to be updated with the non-nightly image tag as well as the new UNSECURED environment variable that's necessary for running the image.