Dependency Injection example 3 - CurrentDomain.GetAssemblies()

Looking at https://github.com/dodyg/practical-aspnetcore/blob/net6.0/projects/dependency-injection/dependency-injection-3/Program.cs - as far as I know this code
```
var types = System.AppDomain.CurrentDomain.GetAssemblies()
.SelectMany(x => x.GetTypes())
.Where(p => type.IsAssignableFrom(p) && p.IsClass);
```
will find only types in assemblies loaded to *this moment*. For perf reason, .NET loads assemblies only when needed, so `GetAssemblies()` is often only a subset of all the app dependencies/assemblies.

A couple of options (not sure if there are more or if there's a way to force all assemblies to be discoverable):
1. introducing static references to all assemblies, so they are loaded when the app starts
2. scanning the folder and manually load each assembly before calculating `types` (caching the result if possible, so this is done only once or on demand)