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

• [x] 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
• [x] 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 :white_check_mark:

Comparison is base (ef4e5ed) 100.00% compared to head (3275d1d) 100.00%.

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.