Add notification in SDK for uploaded files

[ValentaTomas]

When you upload a file via URL on the frontend you have to manually call your backend if you want to react there to this event.

We could have a hook/callback in the SDK that automatically creates these sandbox-related notifications.

This hook could be either an instance or class method:

1. As instance method — requiring you to have a sandbox connection:

sandbox.filesystem.onFileUpload((path) => {})

This could be implemented via watchDir functionality relatively easily.

2. As a class method — this would allow you to have the hook without a sandbox connection:

Sandbox.onFileUpload(sandboxID, (path) => {})

Implementing this is noticeably harder — it would require us to have a notification system/infra and changes to the envd to allow forwarding events to this system.

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The current solution is for example to call the server from the frontend after POSTing to the fileURL, or POSTing to the server and calling sandbox.uploadFile from there.