Add allowsAutomaticUpdates property

[zorgiepoo]

This allows us to determine if automatic downloading/installing of updates option (typically a checkbox) should be enabled or not. This property is based on whether or not automatically checking of updates is enabled, or the SUAllowsAutomaticUpdates option (if specified by developer for custom needs).

The update alert now updates hiding/unhiding the automatic downloading checkbox if the automatic update checking option changes.

The test application is now updated to use -[SPUUpdater allowsAutomaticUpdates] for enabling/disabling the automatic downloads update checkbox.

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested enabling/disabling automatic checking and automatic downloading options in the test app settings window, by defaults CLI, and from changing latter option from the update alert window.

macOS version tested: 26.1 (25B78)