Pdf to image custom page selection (#2576)

# Description

Implemented custom page selection for the pdf-to-image feature, allowing
users to specify which PDF pages to convert to images.

1. Variable Renaming: Changed singleOrMultiple to imageResultType
because it supports three options: single, multiple, and custom.
2. New Field: Added pageNumbers to accept user-defined page selections.
This field appears only when custom is selected in the UI.
3. New Method: Added getPageIndicesToConvert to process and validate the
specified page numbers.
4. Method Update: Updated convertFromPdf to handle custom page numbers,
ensuring only selected pages are converted.
5. Translation Properties: Added two new English translation properties,
custom and customPageNumber, to all language files with placeholder
values. These will need to be translated into country-specific languages
in the future.

Note: If an invalid page number is provided (zero, negative, or exceeds
page count), a single image containing all PDF pages is generated.

Closes #918 

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [x] I have attached images of the change if it is UI based
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [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)

![Screenshot 2025-01-02 at 12 31
29 AM](https://github.com/user-attachments/assets/c4ba3f31-5dd6-4a17-991e-51b86c2eb466)
![Screenshot 2025-01-02 at 12 31
49 AM](https://github.com/user-attachments/assets/3e800a95-2088-4f69-8a01-bd03d7b9e471)

---------

Co-authored-by: Sai Kumar J <saikumar@Sais-MacBook-Air.local>
Co-authored-by: Ludy <Ludy87@users.noreply.github.com>
Co-authored-by: saikumar <saikumar.jetti@gmail.com>
Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>

src/main/resources/templates/convert/pdf-to-img.html

@@ -39,6 +39,12 @@
                     <option value="single" th:text="#{pdfToImage.single}"></option>
                   </select>
                 </div>
+
+                <div class="mb-3" id="singleOptionSection" >
+                  <label for="pageNumbers" th:text="#{pageSelectionPrompt}"></label>
+                  <input type="text" name="pageNumbers" class="form-control" id="pageNumbers" value="all"
+                         th:placeholder="#{pdfToImage.placeholder}" required>
+                </div>
                 <div class="mb-3">
                   <label th:text="#{pdfToImage.colorType}"></label>
                   <select class="form-control" name="colorType">
@@ -59,5 +65,6 @@
       </div>
       <th:block th:insert="~{fragments/footer.html :: footer}"></th:block>
     </div>
+
   </body>
 </html>