feat(domain): add common base classes and interfaces

- Add BaseEntity with audit fields (CreatedDate, CreatedBy, ModifiedDate, ModifiedBy)
- Add IAggregateRoot marker interface for DDD aggregate roots
- Add ValueObject base class with equality comparison by value

These classes provide the foundation for all domain entities and value objects.

src/DigitalData.EmailProfiler.Domain/Common/BaseEntity.cs

@@ -0,0 +1,9 @@
+namespace DigitalData.EmailProfiler.Domain.Common;
+
+public abstract class BaseEntity
+{
+    public DateTime? CreatedDate { get; set; }
+    public string? CreatedBy { get; set; }
+    public DateTime? ModifiedDate { get; set; }
+    public string? ModifiedBy { get; set; }
+}