Refactor downloaders to use ByteDocument and add options builders

Replaces generic RawType with ByteDocument in downloaders and context classes, simplifying type usage. Adds builder classes for FailurePredicateOptions, FragmentOptions, SkipPredicateOptions, and UnitDownloaderOptions to improve configuration flexibility. Introduces DownloadTarget enum and SkipPredicate delegate for more granular download control. Refactors Fluent API interfaces and implementations to remove RawType generics and streamline usage. Adds Playwright and Stealth download strategies for extensibility.

Beam.Downloaders/FragmentOptions.cs

@@ -0,0 +1,7 @@
+namespace Beam.Downloaders;
+
+public record class FragmentOptions {
+    public required int FragmentSize { get; init; }
+    public bool DownloadInParallel { get; init; } = false;
+    public int? ParallelThreads { get; init; }
+}