Support adding attachments to e-mails

What would you like to be added:

Uno Platform implementation of EmailManager currently does not support e-mail attachments. The problem is that Andorid and iOS support file-based attachments only, however UWP API is fully based on streams. There would be two options:

• Take the stream, write to file, use that file as an attachment, then clean it up "sometime" afterward - as it means the API would eat up storage space, which could potentially be in MB range, and there would have to be a check to clean it up even in case something fails. Ideally the file should be stored in the ApplicationData.Current.TemporaryFolder, as that one should get cleaned up by the system automatically
• Add some Uno-specific methods, that would allow adding file-based attachments - this API could be either Android and iOS specific, or it could exist as a universal extension method, that would work on UWP as well, so it could be truly cross-platform.

Why is this needed:

To send files with e-mail. It is often used for sending crash-dumps.

For which Platform:

• iOS
• Android
• WebAssembly
• WebAssembly renderers for Xamarin.Forms
• macOS
• Windows
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