Backup agents can now report upload progress

The BackupAgent.async_upload_backup method now receives a new on_progress callback parameter. Backup agents can call this callback periodically during upload to report the number of bytes uploaded so far:

class ExampleBackupAgent(BackupAgent):

    async def async_upload_backup(
        self,
        *,
        open_stream: Callable[[], Coroutine[Any, Any, AsyncIterator[bytes]]],
        backup: AgentBackup,
        on_progress: OnProgressCallback,
        **kwargs: Any,
    ) -> None:
        """Upload a backup."""
        ...
        bytes_uploaded = 0
        async for chunk in await open_stream():
            await do_upload(chunk)
            bytes_uploaded += len(chunk)
            on_progress(bytes_uploaded=bytes_uploaded)
        ...

The backup manager uses these progress reports to fire UploadBackupEvent events, enabling the frontend to display real-time upload progress to the user.

Check the backup agent documentation for more details.