Replace MvxIoCProvider with Microsoft.Extensions.DependencyInjection
We often get requests to improve the feature set of the IoC Provider we have in MvvmCross. In order to not reinvent the wheels every time, we should switch to Microsoft.Extensions.DependencyInjection.Abstractions and let the users chose any of the supported IoC Providers it supports, which provides the functionality.

This way we can let the consumer chose whether to use AutoFac, Ninject, MS.Extensions.DI or whatever IoC Provider they chose.

- [ ] Remove IMvxIoCProvider
- [ ] Remove MvxSingleton
- [ ] Think about whether to keep the Plugin structure for MvvmCross or let users manage registrations for similar functionality themselves