Move AIFunctionFactory down to M.E.AI.Abstractions (dotnet#6412)

* Remove AIFunctionFactory dependency on M.E.DI

This means reverting the recent changes to it that:
- Special-cased KeyedServices
- Special-cased IServiceProviderIsService
- Used ActivatorUtilities.CreateInstance

* Move AIFunctionFactory down to M.E.AI.Abstractions

* Add CreateInstance delegate to AIFunctionFactoryOptions

To enable use of ActivatorUtilities.CreateInstance or alternative.

* Add some comments

Author: stephentoub