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/resources/messages_en_GB.properties

@@ -525,6 +525,10 @@ home.addImage.title=Add image
 home.addImage.desc=Adds a image onto a set location on the PDF
 addImage.tags=img,jpg,picture,photo
 
+home.attachments.title=Add Attachments
+home.attachments.desc=Add or remove embedded files (attachments) to/from a PDF
+attachments.tags=embed,attach,file,attachment,attachments
+
 home.watermark.title=Add Watermark
 home.watermark.desc=Add a custom watermark to your PDF document.
 watermark.tags=Text,repeating,label,own,copyright,trademark,img,jpg,picture,photo
@@ -1205,6 +1209,12 @@ addImage.everyPage=Every Page?
 addImage.upload=Add image
 addImage.submit=Add image
 
+#attachments
+attachments.title=Add Attachments
+attachments.header=Add attachments
+attachments.description=Allows you to add attachments to the PDF
+attachments.descriptionPlaceholder=Enter a description for the attachments...
+attachments.addButton=Add Attachments
 
 #merge
 merge.title=Merge
@@ -1594,6 +1604,7 @@ fileChooser.dragAndDropPDF=Drag & Drop PDF file
 fileChooser.dragAndDropImage=Drag & Drop Image file
 fileChooser.hoveredDragAndDrop=Drag & Drop file(s) here
 fileChooser.extractPDF=Extracting...
+fileChooser.addAttachments=drag & drop attachments here
 
 #release notes
 releases.footer=Releases