Add Attachments Feature (#3781)

# Description of Changes
Added a new feature to add attachments to a PDF document. 

### Added:
- `AttachmentController`: Endpoint for adding attachments at
`/add-attachments` with parameters `fileInput` for the PDF and
`attachments` as a list of files to attach
- `AttachmentServiceInterface`
- `AttachmentService`: Handles the logic of adding attachments to the
PDF
- `AttachmentUtils`: to handle setting the catalog viewer preferences in
the viewer
- Add Attachments page
- Tests for new feature

### Changes:
- `EmlToPdf`: Moved setting of viewer preferences to `AttachmentUtils`
- `EndpointConfiguration: Included '/add-attachments'
- Updated language files with attachments copy
- General clean up

Closes #1259 

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [x] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [x] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

![348abcd235de976c347b8cd2c7979318](https://github.com/user-attachments/assets/56c3c992-7ab2-4723-8ddb-57038b557287)
![Screenshot 2025-06-20 at 13 52
53](https://github.com/user-attachments/assets/dc7a69c3-a85d-41c4-bac6-6a5d901459b9)
![Screenshot 2025-06-20 at 13 57
55](https://github.com/user-attachments/assets/8025a659-db7e-4441-946c-ce10414974ce)
![Screenshot 2025-06-20 at 13 58
20](https://github.com/user-attachments/assets/ae8d464e-1c95-4db1-92e6-1cf894fa4e83)

- [x] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

stirling-pdf/src/main/java/stirling/software/SPDF/model/api/misc/AddAttachmentRequest.java

@@ -0,0 +1,23 @@
+package stirling.software.SPDF.model.api.misc;
+
+import java.util.List;
+
+import org.springframework.web.multipart.MultipartFile;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+import stirling.software.common.model.api.PDFFile;
+
+@Data
+@EqualsAndHashCode(callSuper = true)
+public class AddAttachmentRequest extends PDFFile {
+
+    @Schema(
+            description = "The image file to be overlaid onto the PDF.",
+            requiredMode = Schema.RequiredMode.REQUIRED,
+            format = "binary")
+    private List<MultipartFile> attachments;
+}