Make date model binding more flexible

An IModelBinderProvider is configured that will attempt to model bind
all Date, DateTime and DateOnly instances in a way that works with the
Date input component. However, sometimes apps pass dates around without
using the Date input component (e.g. as a query parameter) and these
model binders are blocking the built-in ones from running and working.

This exposes a method on our options class to create one of our model
binders. With this, consumers mix in our model binders with their own as
they see fit.

src/GovUk.Frontend.AspNetCore/GovUkFrontendAspNetCoreOptions.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Collections.Generic;
 using GovUk.Frontend.AspNetCore.HtmlGeneration;
+using GovUk.Frontend.AspNetCore.ModelBinding;
 using GovUk.Frontend.AspNetCore.TagHelpers;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
 
 namespace GovUk.Frontend.AspNetCore
 {
@@ -75,5 +77,35 @@ public GovUkFrontendAspNetCoreOptions()
         /// The default is <c>true</c>.
         /// </remarks>
         public bool PrependErrorToTitle { get; set; }
+
+        /// <summary>
+        /// Creates an <see cref="IModelBinder"/> for Date input components using the <see cref="DateInputModelConverter"/>
+        /// registered in <see cref="DateInputModelConverters"/> for the specified <see cref="Type"/>.
+        /// </summary>
+        /// <param name="dateModelType">The type that the created <see cref="IModelBinder"/> should support.</param>
+        /// <returns>
+        /// A <see cref="IModelBinder"/>
+        /// or <see langword="null"/> if there is no <see cref="DateInputModelConverter"/> registered that supports <paramref name="dateModelType"/>.
+        /// </returns>
+        public IModelBinder? GetDateInputModelBinder(Type dateModelType)
+        {
+            foreach (var converter in DateInputModelConverters)
+            {
+                if (converter.CanConvertModelType(dateModelType))
+                {
+                    return GetDateInputModelBinder(converter);
+                }
+            }
+
+            return null;
+        }
+
+        /// <summary>
+        /// Creates an <see cref="IModelBinder"/> for Date input components using the specified <see cref="DateInputModelConverter"/>.
+        /// </summary>
+        /// <param name="converter">The <see cref="DateInputModelConverter"/> to use to convert to and from a <see cref="Date"/>.</param>
+        /// <returns>A <see cref="IModelBinder"/>.</returns>
+        public IModelBinder GetDateInputModelBinder(DateInputModelConverter converter) =>
+            new DateInputModelBinder(converter);
     }
 }

src/GovUk.Frontend.AspNetCore/ModelBinding/DateInputModelBinderProvider.cs

@@ -4,30 +4,21 @@ namespace GovUk.Frontend.AspNetCore.ModelBinding
 {
     internal class DateInputModelBinderProvider : IModelBinderProvider
     {
-        private readonly DateInputModelConverter[] _dateInputModelConverters;
+        private readonly GovUkFrontendAspNetCoreOptions _options;
 
         public DateInputModelBinderProvider(GovUkFrontendAspNetCoreOptions options)
         {
             Guard.ArgumentNotNull(nameof(options), options);
 
-            _dateInputModelConverters = options.DateInputModelConverters.ToArray();
+            _options = options;
         }
 
         public IModelBinder? GetBinder(ModelBinderProviderContext context)
         {
             Guard.ArgumentNotNull(nameof(context), context);
 
             var modelType = context.Metadata.UnderlyingOrModelType;
-
-            foreach (var converter in _dateInputModelConverters)
-            {
-                if (converter.CanConvertModelType(modelType))
-                {
-                    return new DateInputModelBinder(converter);
-                }
-            }
-
-            return null;
+            return _options.GetDateInputModelBinder(modelType);
         }
     }
 }