refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Dynamic/DropDownDataProvider.cs

@@ -11,8 +11,7 @@ using System.Threading.Tasks;
 namespace Beam.Dynamic {
     public class DropDownDataProvider
             : IDataProvider<string>, 
-              IDataProvider<string[]>, 
-              IDataProvider<SourceLink[]> {
+              IDataProvider<string[]> {
         public IBinding? Content { get; set; }
         public string? RelativeTo { get; set; } 
 
@@ -27,26 +26,24 @@ namespace Beam.Dynamic {
             return @base + '/' + relative;
         }
 
-        public SourceLink[] Get(HtmlDocument document) {
+        public string[] Get(HtmlDocument document) {
             if (Content is null)
                 return [];
             var node = Content.Select(document);
             if (node is null)
                 return [];
-            List<SourceLink> links = [];
+            List<string> links = [];
             foreach (var child in node.ChildNodes.Where(x => x.Name == "option")) {
                 var childValue = child.GetAttributeValue("value", null);
                 if (!Uri.TryCreate(GetAbsolute(RelativeTo, childValue), UriKind.Absolute, out _))
                     continue;
-                links.Add(new SourceLink(GetAbsolute(RelativeTo, childValue)));
+                links.Add((GetAbsolute(RelativeTo, 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));