Bff add aspire to samples

[Erwinvandervalk]

This PR adds Aspire to the samples project.

This is part of a journey to improve the development and testing story for the BFF.
By adding aspire, you can now launch all hosts in one go. A couple of things are not yet working:

• Adding aspire projects to the sln
• configuring aspire projects to use the new solutions
• configure multi-targeting .net 8 and 9. This is needed because I ran into this issue: Solution dependency leads to error MSB4057: The target "GetTargetPath" does not exist in the project dotnet/msbuild#4303
• re-write the program.cs to use top level statements. This allows the use of aspire's service-defaults.