Expose UserManager methods wrapped by AspNetIdentityManagerService class

[BruceHunter]

I think it would be a good idea to expose 3 of the lower level methods from the UserManager class wrapped by the AspNetIdentityManagerService class.

Methods to expose (FindByEmailAsync, FindByNameAsync and GeneratePasswordResetTokenAsync).

With these methods one can create a Service that utilizes AspNetIdentityManagerService and build a ResetPassword set of endpoints. Also, IdentityManager (GUI) may use them one day.

I've already done the work in a PR.

Let me know what you think.