Add hardware requirements element

[zorgiepoo]

The first requirement we support is arm64 which allows updates to mark themselves as Apple silicon only.

The message that shows up when users check for updates tells them their Mac is too old and a newer Apple silicon Mac is required for new updates. This introduces a new localizable string.

This requirement is also inferred if the minimum system version is >= 27.0

generate_appcast is also updated to handle a new branch for hardwareRequirements element, and for automatically populating it if needed.

Fixes #2796

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 unit tests on both AS and Intel Macs
• Tested test app by adding <sparkle:hardwareRequirements>arm64</sparkle:hardwareRequirements> on AS (where it's no-op) and Intel Macs.
• Tested running generate_appcast on updates that have only arm64 mach-o slice, and updates that have >= 27.0 minimum system version, and updates that have neither requirement.
• Tested encoding/decoding of hardwareRequirements in SUAppcastItem and SUAppcastItemState

macOS version tested: 26.1 (25B78)