fix: add items key for objc

[zhu-xiaowei]

Description

1. add ClickstreamItemKey for Objective-C API

General Checklist

• Added new tests to cover change, if needed
• Build succeeds using Swift Package Manager
• All unit tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (648f658) 95.50% compared to head (f3dd659) 95.50%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   95.50%   95.50%           
=======================================
  Files          35       35           
  Lines        1937     1937           
=======================================
  Hits         1850     1850           
  Misses         87       87           

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