Big cleanup and adding IdentityBuilder extensions the same way as the EF IdentityBuilder extensions do, allowing customer Identity setup.

src/PasswordHasherOptionsAccessor.cs

@@ -0,0 +1,21 @@
+using Microsoft.Extensions.Options;
+using Microsoft.AspNetCore.Identity;
+
+namespace AspNetCore.Identity.MongoDbCore
+{
+    /// <summary>
+    /// Represents the password hashing options
+    /// </summary>
+    public sealed class PasswordHasherOptionsAccessor : IOptions<PasswordHasherOptions>
+    {
+        /// <summary>
+        /// Gets options which use the IdentityV3 compat mode, and set the iteration count to 200000 PBKDF2-SHA256 iterations 
+        /// (roughly 200ms of work)
+        /// </summary>
+        public PasswordHasherOptions Value { get; } = new PasswordHasherOptions
+        {
+            CompatibilityMode = PasswordHasherCompatibilityMode.IdentityV3,
+            IterationCount = 200000
+        };
+    }
+}