fix(ios): revert accidental change to category "callback"

[erisu]

Motivation and Context

Fix an accidental breaking property name change from callback to identifier.
Fix issue where category's callback was not being property set and triggered.

Description

In the documentation and typings, the CategoryActionData object should include a key named callback.

On iOS, the native code refers to callback as identifier. Because of this, the native implementation was mistakenly modified to extract the callback value using the incorrect key identifier.

This PR reverts the key back to callback to align with the documentation and typings.

The issue was discovered during testing of the typings.

Updated code to make sure that the actionCallback is not removed in cases where the value is not default.

Related Issue

n/a

How Has This Been Tested?

• Confirmed that the callback is triggered when clicking the category option.
• Tested the default, category, & actionCallback case while app state is in Foreground, Background, & Inactive

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.