Introduced interface on BackOfficeUserManager (#8913)

* Introduced IBackOfficeUserManager

* Fixed test

* Moved class into own file

Co-authored-by: Elitsa Marinovska <elm@umbraco.dk>

src/Umbraco.Web.BackOffice/Extensions/BackOfficeServiceCollectionExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Identity;
@@ -57,7 +58,7 @@ namespace Umbraco.Extensions
             services.BuildUmbracoBackOfficeIdentity()
                 .AddDefaultTokenProviders()
                 .AddUserStore<BackOfficeUserStore>()
-                .AddUserManager<BackOfficeUserManager>()
+                .AddUserManager<IBackOfficeUserManager, BackOfficeUserManager>()
                 .AddSignInManager<BackOfficeSignInManager>()
                 .AddClaimsPrincipalFactory<BackOfficeClaimsPrincipalFactory<BackOfficeIdentityUser>>();