fix: add user_first_touch_timestamp user attribute in _first_open event

[zhu-xiaowei]

Description

1. add user_first_touch_timestamp in _first_open event

General Checklist

• Added new tests to cover change, if needed
• 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 (ef4e5ed) 100.00% compared to head (3275d1d) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #34   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        24           
  Lines          995       995           
  Branches       183       183           
=========================================
  Hits           995       995           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.