Add support for DbContextPool

[iwedaz]

Current implementation don't find user implemented DbContext that injected with AddDbContextPool().

If you compare different versions of AddDbContext() and AddDbContextPool() extension methods, you will see all of them have the same registration for scoped user DbContext implementation.

EF Core 6 AddDbContextPool AddDbContext

EF Core 7 AddDbContextPool AddDbContext

EF Core 8 AddDbContextPool AddDbContext

Therefor this approach for searching user implemented DbContext registrations is more universal.