feat(amplify_push): add example app

[ekjotmultani]

Runnable example app for amplify_push. Stacked on #6985 (targets feat/amplify-push-v3), so the diff shows only the example app; rebased onto main once #6985 merges.

Exercises the push APIs on device: permission request/status, live device token, foreground/opened/launch notification display with an on-screen log, iOS badge. Includes a google-services placeholder and a README for sending a test push.

Verified locally: example widget test passes; analyzer clean; iOS AmplifyUtilsNotifications podspec resolves.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.23%. Comparing base (48cb67a) to head (956a7ec).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           feat/amplify-push-v3    #7079   +/-   ##
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  Coverage                 43.23%   43.23%           
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  Files                        99       99           
  Lines                      7769     7769           
  Branches                   3401     3401           
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  Hits                       3359     3359           
  Misses                     4410     4410           

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