Ensure all service methods are marked as virtual

[AndreiMaz]

There are several methods that are not marked as virtual. Check the Nop.Services and other core libraries. We can find that by "public async" keyword

Forum source: https://www.nopcommerce.com/en/boards/topic/102451/any-specific-reasons-for-some-of-the-methods-in-productservicecs-not-being-marked-as-virtual-methods

[dahalsanju741]

May need to verify that for protected methods as well; otherwise, those won't be able to be overridden either.
Example GetFromPriceAsync method from ProductModelFactory class

[skoshelev]

Closed #7840