Add configurable FlutterSecureStorage to FlutterSecureCredentialsStorage

[Copilot]

The FlutterSecureCredentialsStorage class previously created the underlying FlutterSecureStorage with default options, making it impossible to configure platform-specific settings like KeyChain accessibility on iOS.

This PR adds an optional storage parameter to the constructor, allowing users to provide a custom FlutterSecureStorage instance with specific configuration options:

// Default usage (unchanged)
const storage = FlutterSecureCredentialsStorage();

// Custom configuration with iOS KeyChain accessibility
const customStorage = FlutterSecureCredentialsStorage(
  storage: FlutterSecureStorage(
    iOptions: IOSOptions(
      accessibility: KeychainAccessibility.first_unlock_this_device,
    ),
    aOptions: AndroidOptions(
      encryptedSharedPreferences: true,
    ),
  ),
);

Key features:

• Fully backward compatible - existing code works unchanged
• Enables platform-specific configurations for Android, iOS, macOS, Linux, Windows, and Web
• Well-documented with clear usage examples
• Comprehensive test coverage

The implementation uses a private field to store the custom storage and a getter that falls back to the default instance when no custom storage is provided, ensuring minimal code changes while preserving all existing functionality.

Fixes #478.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• dl-ssl.google.com
  • Triggering command: curl -fsSL REDACTED (dns block)
• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)
• https://api.github.com/repos/flutter/flutter/releases/latest
  • Triggering command: curl -s REDACTED (http block)
• https://storage.googleapis.com/flutter_infra_release/flutter/1e9a811bf8e70466596bcf0ea3a8b5adb5f17f7f/dart-sdk-linux-x64.zip
  • Triggering command: curl --retry 3 --continue-at - --location --output /tmp/flutter/bin/cache/dart-sdk-linux-x64.zip REDACTED (http block)
• https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.24.3-stable.tar.xz
  • Triggering command: wget -q REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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[PiotrRogulski]

@copilot Update the changelog and the package version

[Copilot]

@copilot Update the changelog and the package version

Updated package version to 3.1.0 and added changelog entry describing the new configurable FlutterSecureStorage feature. Commit 784bcd9