Add Blazor Sample for loading plugins

Since Blazor offers such a modularity, why not have a good sample showing how components can be loaded/unloaded as plugins into a host application.

Yes, there's some projects that offer that but it would be nice to have an official sample on how to do it.

Please update [https://github.com/dotnet/samples/tree/main/core/extensions/AppWithPlugin](https://github.com/dotnet/samples/tree/main/core/extensions/AppWithPlugin) with such a sample