Implement new .NET 8 IKeyedServiceProvider interface

[elca-lan]

With .NET 8 a new interface was added that should(?) be implemented by service providers, the IKeyedServiceProvider

Explanations and documentation:

• https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-8.0#keyed-services
• https://andrewlock.net/exploring-the-dotnet-8-preview-keyed-services-dependency-injection-support/
• https://weblogs.asp.net/ricardoperes/net-8-dependency-injection-changes-keyed-services

We should implement that interface too. It should be relatively easy to do this with a binding constraint since we already store the entire ServiceDescriptor in the binding metadata. From there we can access the ServiceDescriptor.ServiceKey and find the exact binding.

[DominicUllmann]

See #18