Add advanced builders for search and buttons in DataTables

[wlady2906]

Detailed configuration methods are added for Search and button collections (CollectionButton) in the DataTables builders. Includes new classes SearchingBuilder, CollectionButtonBuilder, LayoutOptionsBuilder, LayoutOptions, and Searching for advanced search and layout options. The DatatableJs model now supports the Search property. The example in Index.cshtml is updated to show advanced layouts with buttons and custom search.

//index.cshtml
.Layout(x =>
        {
            x.Position("topStart", top =>
            {
                top.Button(btn => btn.Text("Custom button").Action(@$"function(){{ alert('hello from layout options')}}"));
                top.Buttons("selectAll", "selectNone");
                top.Button(btn => btn.Text("Page length").Extend("pageLength"));
                top.Button(btn => btn.Text("Column visibility").Extend("colvis"));
                top.Button(btn =>
                {
                    btn.Text("Custom Export").Extend("collection").Buttons(btn =>
                    {
                        btn.Button(p => p.Text("PDF"));
                        btn.Button(p => p.Text("Excel"));
                    });
                });
            });
            x.Position("topEnd", end =>
            {
                end.Search(s => s.Placeholder("Type here for search..."));
            });
            x.Set("bottomStart", "info");
            x.Set("bottomEnd", "paging");
        })                         

Output:

[ekondur]

Dereferences a null list in Buttons(params string[] buttons). LayoutsBuilder.Position() creates a fresh LayoutOptions, and its Buttons field starts as null LayoutOptions.cs (line 14). The PR’s sample uses this path directly in Index.cshtml (line 68), so top.Buttons("selectAll", "selectNone") will throw a server-side NullReferenceException before the page renders. Even if the list were initialized, _options.Buttons.Add(buttons) would serialize as a nested array, while DataTables expects a flat buttons list.

https://datatables.net/reference/option/buttons.buttons

[ekondur]

Searching.cs (line 14) models smart and caseInsensitive as non-nullable bool values defaulting to false. The serializer only ignores null values RKHelperExtensions.cs (line 133), so even a minimal search config such as the new placeholder example in Index.cshtml (line 82) will emit smart:false and caseInsensitive:false. That silently changes DataTables’ default search behavior.
Default values are true:
https://datatables.net/reference/option/search.smart and https://datatables.net/reference/option/search.caseInsensitive

[ekondur]

SearchingBuilder.cs (line 37) and Searching.cs (line 19) model search.regex as a string, but DataTables expects a boolean. With the current API, callers cannot produce the correct shape like regex: true; they can only emit string values.
https://datatables.net/reference/option/search.regex