chore: fix/update example & integration CI

[techouse]

• [x] fixed integration Flutter example
• [x] migrated Flutter Driver test to integration_test
• [x] updated iOS example
• [x] added iOS integration test
• [x] updated Android example
  • supersedes https://github.com/mogol/flutter_secure_storage/pull/724
• [x] updated Android integration test
• [x] updated Web example
• [x] updated macOS example

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Addresses comment https://github.com/mogol/flutter_secure_storage/pull/719#issuecomment-2124004338

[techouse]

@juliansteenbakker I've made quite a few changes, so please let me know if you have any questions.

[juliansteenbakker]

Thank you!