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/IArticleData.cs

@@ -0,0 +1,10 @@
+namespace Beam.Abstractions;
+
+public interface IArticleData : IDocumentMetaData, IEquatable<IArticleData?> {
+    string? Name { get; set; }
+    string[] Authors { get; set; }
+    string? Language { get; set; }
+    string[] Categories { get; set; }
+    string? Version { get; set; }
+    string? Description { get; set; }
+}