AddDbContext method for DI

In general we can try to resolve the following types in order:
1. ```IOptionsAccessor<DbContextOptions<TContext>>
2. ```IOptionsAccessor<DbContextOptions>
3. ```DbContextOptions<TContext>
4. ```DbContextOptions

Case 1 is the one that is important to resolve even if the constructor doesn't explicitly take it, because that is what AddDbContext makes available. I would be fine with supporting the others only via an explicit constructor, but I would like to hear what @ajcvickers and @rowanmiller think about it.