iOS: expose onStart hook

[nancywu1]

pull this change out of #582

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.93%. Comparing base (598509c) to head (eb1e177).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #612      +/-   ##
==========================================
+ Coverage   89.92%   89.93%   +0.01%     
==========================================
  Files         335      336       +1     
  Lines       20419    20434      +15     
  Branches     1956     1956              
==========================================
+ Hits        18361    18377      +16     
+ Misses       2044     2043       -1     
  Partials       14       14              

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