Implement sharing API

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

At least iOS, Android and macOS seem to offer an API to share a document with another app. As seen in:

iOS | Android | macOS
:-------------------------:|:-------------------------:|:-------------------------:
| | 

Toga doesn't currently offer that.

I'm writing an app that produces a file, and it would be handy if the user could simply share this file with other apps, directly from my app.

### Describe the solution you'd like

Implement sharing API, that allows us to present the user with the option to share a document/file with another app.

### Describe alternatives you've considered

None

### Additional context

_No response_

No files, tests, or entry points are named. Start by surveying Toga's existing APIs and its iOS, Android, and macOS platform implementations, then compare each platform's document-sharing mechanism; done means a consistent API lets an app present a file-sharing option on the supported platforms.

Written by the indexing model from the issue text.