feat: support lazy initialization of the SDK

[zhu-xiaowei]

• PR title and description conform to Pull Request guidelines.

Description of changes:

1. support lazy initialization of the SDK and will also send _first_open, _app_start and _session_start events.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests

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 ✅

Project coverage is 86.70%. Comparing base (60857e5) to head (07c7903).
Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #74      +/-   ##
============================================
+ Coverage     86.50%   86.70%   +0.19%     
- Complexity      458      468      +10     
============================================
  Files            42       42              
  Lines          1690     1707      +17     
  Branches        153      156       +3     
============================================
+ Hits           1462     1480      +18     
  Misses          150      150              
+ Partials         78       77       -1     

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