[Request]: add a saveFiles option to share_plus

### Plugin

share_plus

### Use case

The share_plus plugin already has most of the infrastructure to support saving files to a local storage.
It would be great if `ShareParams` had an option that would allow to open a platform-specific file/document UI to save file(s).

### Proposal

The platform's share dialog on iOS has an option to save files. But on MacOS and Android there is no such option available and we have to resort to using half-broken 3rd party plugins that have issues, e.g. unable to share/save very large files, which the `share_plus` already worked out.

The proposal is to add an option, perhaps a boolean `saveFiles` flag to the `ShareParams` and use that flag on Android and other platform to open the file/document selection UI and save files to the local storage instead of using platform sharing options.

I can try to submit a PR to add support of such option for Android and maybe/hopefully for iOS/macOS.

Inizia individuando ShareParams e gli attuali punti di ingresso della piattaforma di share_plus per Android, iOS e macOS; confronta il modo in cui ciascuna piattaforma gestisce attualmente i file e la relativa interfaccia di salvataggio. Il lavoro è completo quando viene definito il comportamento dell'opzione su queste piattaforme e viene aperta l'interfaccia appropriata per file o documenti per salvare i file selezionati.

Scritto dal modello di indicizzazione a partire dal testo della issue.