Add Luna.Shared library with shared DTOs and models

Introduce ChannelMessage and ChatStreamUpdate types used across the
solution for inter-project communication between channels, core, and
the web interface.

Luna.Shared/ChannelMessage.cs

@@ -0,0 +1,10 @@
+namespace Luna.Shared;
+
+public record ChannelMessage(
+    string MessageId,
+    string ConversationId,
+    string SenderId,
+    string SenderName,
+    string Content,
+    DateTimeOffset Timestamp,
+    Dictionary<string, string>? Metadata = null);