[Greenfield] Allow passing email instead of user id in API (#4732)

BTCPayServer/UserManagerExtensions.cs

@@ -0,0 +1,19 @@
+#nullable enable
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Identity;
+
+namespace BTCPayServer
+{
+    public static class UserManagerExtensions
+    {
+        public async static Task<TUser?> FindByIdOrEmail<TUser>(this UserManager<TUser> userManager, string? idOrEmail) where TUser : class
+        {
+            if (string.IsNullOrEmpty(idOrEmail))
+                return null;
+            if (idOrEmail.Contains('@'))
+                return await userManager.FindByEmailAsync(idOrEmail);
+            else
+                return await userManager.FindByIdAsync(idOrEmail);
+        }
+    }
+}