Is your feature request related to a problem? Please describe.
We'd like to add support for https://uppy.io/ for file uploading. This would require:

Describe the solution you'd like
Initially, creating an example (similar to other blocknote examples in the example directory would suffice (as part of follow up work, later we could see what we can extract to a reusable package).

The main requirements are:

• Most important: customize the BlockNote FilePanel to use the Uppy React library (use a FilePanelController and the Uppy Dashboard for this)
• Make sure the uploadFile function also uses the uppy pipeline
• It should be clear in the code where you can configure additional Uppy options such as a companion URL or transloadit server

Of course, it must work for selecting / uploading resources for different blocks (e.g.: File / Image / Video)