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.Models/RetryReport.cs

@@ -0,0 +1,14 @@
+using Beam.Abstractions;
+
+namespace Beam.Models {
+    public readonly struct RetryReport  : IRetryReport {
+        public RetryReport(int tryNumber, string link) {
+            TryNumber = tryNumber;
+            Link = link;
+        }
+
+        public int TryNumber { get; }
+        public string Link { get; }
+    }
+
+}