Expose -[SPUStandardUserDriver activeUpdateAlert] as private API

[zorgiepoo]

Related: #1859

I don't want to commit to making any "mandatory update" changes but in the short term if clients want to change the window they have the option to, albeit through "private" means.

Requires clients to #import <Sparkle/SPUStandardUserDriver+Private.h>

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

Only bug fixes to regressions or security fixes are being backported to the 1.x (master) branch now. If you believe your change is significant enough to backport, please also create a separate pull request against the master branch.

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 window controller & alert window is accessible in a test app in -[SPUStandardUserDriverDelegate standardUserDriverWillHandleShowingUpdate:forUpdate:state:]

macOS version tested: 12.5.1 (21G83)