feat: Add onStart lifecycle hook

[Mrtenz]

This adds a new lifecycle hook "onStart", to be called when the client starts. The Snap can use this to perform any initialisation needed, such as setting up a WebSocket connection or background event.

Closes #3453.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (443340f) to head (15812a5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3455   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files         404      404           
  Lines       11317    11334   +17     
  Branches     1764     1766    +2     
=======================================
+ Hits        11115    11132   +17     
  Misses        202      202           

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