Add a performant way to register a context factory in DI

[AndriySvyryd]

This would make using short-lived contexts easier.
Related: #19610

[AndriySvyryd]

See https://github.com/vany0114/EF.DbContextFactory

[jzabroski]

Hi @AndriySvyryd

Currently, we ask each T in an IRepository<T> who their IDbContext is, which requires looping over all entity registrations - so this should solve two problems with one abstraction. This context factory sounds like a nice improvement, and it avoids directly exposing Func<T> which, based on my recent reading of old 2016 conversations between David Fowler and SImpleInjector team, ASP.NET team does not want to do.