Update pageable helpers to assist in new model serialization generation (Azure#38399)

* update pageable helpers to assist in new model serialization generation

* make functions back compat

* keep pageable the same but make the wrapper classes internal

Author: m-nash

sdk/core/Azure.Core/src/Shared/PageableHelpers.cs

@@ -163,7 +163,7 @@ public override IEnumerable<Page<T>> AsPages(string? continuationToken = default
             }
         }
 
-        private class AsyncPageableWrapper<T> : AsyncPageable<T> where T : notnull
+        internal class AsyncPageableWrapper<T> : AsyncPageable<T> where T : notnull
         {
             private readonly PageableImplementation<T> _implementation;
 
@@ -176,7 +176,7 @@ public AsyncPageableWrapper(PageableImplementation<T> implementation)
             public override IAsyncEnumerable<Page<T>> AsPages(string? continuationToken = null, int? pageSizeHint = null) => _implementation.AsPagesAsync(continuationToken, pageSizeHint, default);
         }
 
-        private class PageableWrapper<T> : Pageable<T> where T : notnull
+        internal class PageableWrapper<T> : Pageable<T> where T : notnull
         {
             private readonly PageableImplementation<T> _implementation;
 
@@ -189,7 +189,7 @@ public PageableWrapper(PageableImplementation<T> implementation)
             public override IEnumerable<Page<T>> AsPages(string? continuationToken = null, int? pageSizeHint = null) => _implementation.AsPages(continuationToken, pageSizeHint);
         }
 
-        private class PageableImplementation<T>
+        internal class PageableImplementation<T>
         {
             private readonly Response? _initialResponse;
             private readonly Func<int?, HttpMessage>? _createFirstPageRequest;