feat: Add support for signing macOS PKG installers and implement productbuild functionality

[ahmtydn]

PR: Add support for signing macOS PKG installers and implement productbuild functionality

Description

This PR introduces support for signing macOS PKG installers and implements the necessary productbuild functionality. The changes include:

• Added a wrapper for the productbuild command to create signed PKG installers.
• Updated the macOS release flow to optionally generate a signed PKG installer if a signing identity is provided.
• Extended CLI options to allow specifying a Developer ID Installer identity for PKG signing.
• Added tests for the new productbuild functionality.

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

Files Changed

• packages/shorebird_cli/bin/shorebird.dart
• packages/shorebird_cli/lib/src/commands/release/macos_releaser.dart
• packages/shorebird_cli/lib/src/commands/release/release_command.dart
• packages/shorebird_cli/lib/src/executables/executables.dart
• packages/shorebird_cli/lib/src/executables/productbuild.dart (new)
• packages/shorebird_cli/test/src/executables/productbuild_test.dart (new)

Summary

• Implements signed PKG installer creation for macOS releases.
• Adds CLI option for specifying PKG signing identity.
• Adds productbuild wrapper and related tests.

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Please review and let me know if further changes are needed.

[kingdomseed]

This looks really exciting.