Feature request: a way to void local file storage for file fields

I have custom form actions defined by a plugin that upload files to cloud storage locations. I would like to explicitly **not** store them locally because they are sensitive (I use write-only credentials to upload them).

I looked through `$form->uploadFiles()` and it looks to error out when destination is not set ([around line 604](https://github.com/getgrav/grav-plugin-form/blob/b818031544443be552098e2cc936a53493be5e4b/classes/Form.php#L604)).

I would find it useful to be able to suppress form upload with something like `destination: false`. I couldn't see an obvious elegant patch for this, unfortunately.

I don't know if the use case(s) justify this, but thought I'd ask. I could probably create a custom form action to clean up the uploaded files to work around this limitation.