Ingest ExternalReviews from K+

Adds a new entity ExternalChapterMetadata, which would allow us to
extend chapters to Recommendations, Ratings, etc in the future

API/Entities/Metadata/ExternalChapterMetadata.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+
+namespace API.Entities.Metadata;
+
+/// <summary>
+/// External Metadata from Kavita+ for a Chapter
+/// </summary>
+/// <remarks>
+/// As apposed to <see cref="ExternalSeriesMetadata"/>,
+/// we do not have a ValidUntilUtc, as this is only matched together with the series.
+/// </remarks>
+public class ExternalChapterMetadata
+{
+    public int Id { get; set; }
+
+    public int ChapterId { get; set; }
+
+    public ICollection<ExternalChapterReview> ExternalReviews { get; set; } = null!;
+
+}