refactor: unify binding & data provider interfaces

- Removed BindingType enum and all related logic from Binding.
- Made Binding implement new IBinding and IKeyed interfaces.
- Moved node selection logic to IBinding.Select; removed Resolve* methods from Binding.
- Added new IBinding interface for XPath/CssPath selection.
- Refactored IDataProvider to generic IDataProvider<T>; removed GetNode.
- Updated ListContentDataProvider and ParagraphedContentDataProvider to use IBinding.
- Added new ContentsDataProvider, ContentsArrayDataProvider, and DropDownDataProvider for flexible data extraction.
- Updated DataBindings to use IDataProvider<T> properties instead of Binding.
- Updated all usages to new interfaces and patterns.

Beam.Dynamic/DropDownDataProvider.cs

@@ -0,0 +1,41 @@
+using HtmlAgilityPack;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace Beam.Dynamic {
+    public class DropDownDataProvider
+            : IDataProvider<string>, 
+              IDataProvider<string[]>, 
+              IDataProvider<SourceLink[]> {
+        public IBinding? Content { get; set; }
+
+        public SourceLink[] Get(HtmlDocument document) {
+            if (Content is null)
+                return [];
+            var node = Content.Select(document);
+            if (node is null)
+                return [];
+            List<SourceLink> links = [];
+            foreach (var child in node.ChildNodes.Where(x => x.Name == "option")) {
+                var childValue = child.GetAttributeValue("value", null);
+                if (!Uri.TryCreate(childValue, UriKind.Absolute, out _))
+                    continue;
+                links.Add(new SourceLink(childValue));
+            }
+
+            return links.ToArray();
+        }
+
+        string[] IDataProvider<string[]>.Get(HtmlDocument document) {
+            return this.Get(document).Select(x => x.Link.AbsoluteUri).ToArray();
+        }
+
+        string IDataProvider<string>.Get(HtmlDocument document) {
+            return JsonSerializer.Serialize(this.Get(document));
+        }
+    }
+}