hide delete button based on annotation custom data

[mutumbakato]

This pull request introduces a new feature that allows conditional hiding of the delete button for PDF annotations based on custom data. The implementation covers both Android and iOS platforms, and a new Flutter example demonstrates how to use this feature. The changes include UI logic, platform-specific implementations, and an example for users.

Feature: Hide Delete Button on Annotations

Android Implementation:

• Added logic in FlutterPdfUiFragment to check annotation custom data for a hideDelete property and hide the delete button in the contextual toolbar if present. [1] [2] [3]

iOS Implementation:

• Implemented delegate methods in PspdfkitPlatformViewImpl to filter out the delete action from the annotation menu based on the hideDelete custom data property.

Flutter Example:

• Added a new example (hide_delete_annotation_example.dart) that demonstrates adding protected (non-deletable) and editable (deletable) annotations, and shows how the delete button is conditionally displayed.
• Registered the new example in the app's example list and navigation logic. [1] [2] [3]