Secrets API?

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

Is there a standard way that people handle user provided application secrets (tokens, passwords, etc) in Toga? I recently created a Toga app for macOS and used [keyring](https://github.com/jaraco/keyring), which was great. However, it doesn't work on iOS.

### Describe the solution you'd like

Should Toga have its own secrets API, or officially endorse keyring as the solution and look for someone to contribute iOS and Android support to that project.

### Describe alternatives you've considered

I tried tricking keyring into using its macOS backend (which uses ctypes to talk to the macOS keychain API) on iOS, but that resulted in an error at runtime.

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by reviewing Toga's existing platform APIs and keyring's macOS, iOS, and Android support, then determine whether the project should provide a secrets API or endorse keyring. Done would require an agreed design and a clear cross-platform implementation scope.

Written by the indexing model from the issue text.