Add Secure Enclave Support to iOS/macOS

[ahmeddhus]

Description

This PR adds support for storing keychain items using the Secure Enclave on iOS and macOS.
A new useSecureEnclave boolean option is introduced in IOSOptions and AppleOptions.
When enabled, secrets are stored using Secure Enclave-backed keys if available.
If Secure Enclave is unavailable or unwrapping fails, the plugin falls back to standard keychain storage.

Changes

• Added useSecureEnclave to Dart options (IOSOptions, AppleOptions)
• Passed the flag to native code via method channel
• Updated iOS/macOS native implementations to use Secure Enclave when requested
• Implemented fallback logic for devices without Secure Enclave
• Updated integration tests to cover Secure Enclave and fallback scenarios
• Updated documentation and code comments

Motivation

Addresses #917
Provides stronger security for sensitive data on supported Apple devices.

Checklist

• All tests pass
• Documentation updated
• Backwards compatible

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.78%. Comparing base (71b75a3) to head (649c4e6).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #989      +/-   ##
===========================================
+ Coverage    82.66%   82.78%   +0.11%     
===========================================
  Files            9        9              
  Lines          150      151       +1     
===========================================
+ Hits           124      125       +1     
  Misses          26       26              

Flag	Coverage Δ
unittests	82.78% <100.00%> (+0.11%)	⬆️

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