AddDbContext method for DI

Not sure what information is missing to make a call. We can either pass the option instance to the context constructor as an argument (we get to choose which constructors we have) or have the constructor locate the service in the "magic" path. I am saying that the only case in which it seems compelling to do service location is ```IOptionsAccessor<DbContextOptions<TContext>>``` because that enables a parameterless constructor work with ```AddDbContext<TContext>()```. For all other cases it seems reasonable that customers add constructors that take options and pass them to the base constructors. Those will work with both direct instantiation and DI.