allow for bundling 3rd-party platform-specific frameworks (on macOS)

### What is the problem or limitation you are having?

I want to have the mac version of my apps auto-update with [Sparkle](https://sparkle-project.org), so that it will present a familiar update UX as well as working with 3rd-party tools such as [MacUpdater](https://www.corecode.io/macupdater/); I also have certain other platform-specific functionality I'd like to be able to include, such as [MASShortcut](https://github.com/cocoabits/MASShortcut).

### Describe the solution you'd like

Something like py2app's `frameworks=` option, although possibly labeled in a much more platform-specific way.

### Describe alternatives you've considered

Right now I just use py2app.

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by tracing Briefcase's macOS application bundling flow and compare it with py2app's frameworks option; done should allow platform-specific frameworks such as Sparkle and MASShortcut to be included in the generated macOS app.

Written by the indexing model from the issue text.