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.Abstractions/IDataProvider.cs

@@ -0,0 +1,14 @@
+using HtmlAgilityPack;
+using System.Text.Json.Serialization;
+
+namespace Beam.Dynamic;
+
+public interface IDataProvider {
+    public string GetString(HtmlDocument document)
+        => (this as IDataProvider<object>)?.Get(document)?.ToString() ?? "";
+}
+
+public interface IDataProvider<out T> : IDataProvider {
+    public T Get(HtmlDocument document);
+    //public HtmlNode? GetNode(HtmlDocument document);
+}