Add support for email attachments in messaging service

Introduced the `EmailAttachmentContext` class to represent email
attachments, with properties for file name, content, content type,
inline display behavior, and content ID. Used conditional compilation
to support both .NET Framework and .NET versions.

Updated the `EmailContext` class to include an `Attachments`
property, enabling emails to include attachments as byte arrays or
file paths.

Enhanced the `LimilabsEmailService` class to handle attachments:
- Added the `AddAttachments` method to process inline and regular
  attachments.
- Integrated attachment handling into the email-building process.

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsEmailService.cs

@@ -46,6 +46,8 @@ public class LimilabsEmailService(
             else
                 builder.Text = context.Body;
 
+            AddAttachments(builder, context.Attachments);
+
             var mail = builder.Create();
 
             result = await smtp.SendMessageAsync(mail, cancellationToken);
@@ -108,4 +110,33 @@ public class LimilabsEmailService(
 
         return message.ToString();
     }
+
+    private static void AddAttachments(MailBuilder builder, IEnumerable<EmailAttachmentContext> attachments)
+    {
+        foreach (var attachment in attachments)
+        {
+            if (attachment.IsInline)
+            {
+                var visual = builder.AddVisual(attachment.Content);
+                visual.FileName = attachment.FileName;
+                visual.ContentId = string.IsNullOrWhiteSpace(attachment.ContentId)
+                    ? attachment.FileName
+                    : attachment.ContentId;
+
+                if (!string.IsNullOrWhiteSpace(attachment.ContentType))
+                    visual.ContentType = ContentType.Parse(attachment.ContentType);
+            }
+            else
+            {
+                var part = builder.AddAttachment(attachment.Content);
+                part.FileName = attachment.FileName;
+
+                if (!string.IsNullOrWhiteSpace(attachment.ContentType))
+                    part.ContentType = ContentType.Parse(attachment.ContentType);
+
+                if (!string.IsNullOrWhiteSpace(attachment.ContentId))
+                    part.ContentId = attachment.ContentId;
+            }
+        }
+    }
 }