Refactor SpecialFieldsPreferences to support reset/reload (#14409)

[shubhamk0205]

Closes #14409

Refactored SpecialFieldsPreferences to follow the same pattern as WorkspacePreferences (PR #13894), enabling proper reset and reload functionality. Added a private default constructor, getDefault() static method, and setAll() method to SpecialFieldsPreferences. Updated JabRefGuiPreferences to use the new getSpecialFieldsPreferencesFromBackingStore() method and integrated reset/reload support in clear() and importPreferences() methods.

Steps to test

1. Test preference reset:

   • Open JabRef
   • Go to Preferences and change the "Special Fields" setting (if available in the UI)
   • Reset preferences (via menu or programmatically)
   • Verify that the Special Fields preference resets to its default value (enabled/true)

2. Test preference import:

   • Export your current preferences to a file
   • Modify the exported preferences file to change the specialFieldsEnabled value
   • Import the modified preferences file back into JabRef
   • Verify that the Special Fields preference reflects the imported value

3. Verify normal operation:

   • Ensure that changing the Special Fields preference in the UI still works correctly
   • Verify that the preference persists after restarting JabRef

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license

• [/] I manually tested my changes in running JabRef (always required)

• [/] I added JUnit tests for changes (if applicable)

• [/] I added screenshots in the PR description (if change is visible to the user)

• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)

• [/] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[koppor]

Seems to be good. Requesting a second review.