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.

testing/allEndpointsRemovedSettings.yml

@@ -128,7 +128,7 @@ ui:
   languages: [] # If empty, all languages are enabled. To display only German and Polish ["de_DE", "pl_PL"]. British English is always enabled.
 
 endpoints: # All the possible endpoints are disabled
-  toRemove: [crop, merge-pdfs, multi-page-layout, overlay-pdfs, pdf-to-single-page, rearrange-pages, remove-image-pdf, remove-pages, rotate-pdf, scale-pages, split-by-size-or-count, split-pages, split-pdf-by-chapters, split-pdf-by-sections, add-password, add-watermark, auto-redact, cert-sign, get-info-on-pdf, redact, remove-cert-sign, remove-password, sanitize-pdf, validate-signature, file-to-pdf, html-to-pdf, img-to-pdf, markdown-to-pdf, pdf-to-csv, pdf-to-html, pdf-to-img, pdf-to-markdown, pdf-to-pdfa, pdf-to-presentation, pdf-to-text, pdf-to-word, pdf-to-xml, url-to-pdf, add-image, add-page-numbers, add-stamp, auto-rename, auto-split-pdf, compress-pdf, decompress-pdf, extract-image-scans, extract-images, flatten, ocr-pdf, remove-blanks, repair, replace-invert-pdf, show-javascript, update-metadata, filter-contains-image, filter-contains-text, filter-file-size, filter-page-count, filter-page-rotation, filter-page-size] # list endpoints to disable (e.g. ['img-to-pdf', 'remove-pages'])
+  toRemove: [crop, merge-pdfs, multi-page-layout, overlay-pdfs, pdf-to-single-page, rearrange-pages, remove-image-pdf, remove-pages, rotate-pdf, scale-pages, split-by-size-or-count, split-pages, split-pdf-by-chapters, split-pdf-by-sections, add-password, add-watermark, auto-redact, cert-sign, get-info-on-pdf, redact, remove-cert-sign, remove-password, sanitize-pdf, validate-signature, file-to-pdf, html-to-pdf, img-to-pdf, markdown-to-pdf, pdf-to-csv, pdf-to-html, pdf-to-img, pdf-to-markdown, pdf-to-pdfa, pdf-to-presentation, pdf-to-text, pdf-to-word, pdf-to-xml, url-to-pdf, add-image, add-page-numbers, add-stamp, auto-rename, auto-split-pdf, compress-pdf, decompress-pdf, extract-image-scans, extract-images, flatten, ocr-pdf, remove-blanks, repair, replace-invert-pdf, show-javascript, update-metadata, filter-contains-image, filter-contains-text, filter-file-size, filter-page-count, filter-page-rotation, filter-page-size, add-attachments] # list endpoints to disable (e.g. ['img-to-pdf', 'remove-pages'])
   groupsToRemove: [] # list groups to disable (e.g. ['LibreOffice'])
 
 metrics:

testing/endpoints.txt

@@ -30,6 +30,7 @@
 /api/v1/misc/add-stamp
 /api/v1/misc/add-page-numbers
 /api/v1/misc/add-image
+/api/v1/misc/add-attachments
 /api/v1/convert/url/pdf
 /api/v1/convert/pdf/xml
 /api/v1/convert/pdf/word

testing/webpage_urls.txt

@@ -51,3 +51,4 @@
 /swagger-ui/index.html
 /licenses
 /releases
+/add-attachments