Allow custom asset upload destinations

[devinhalladay]

Is your feature request related to a problem? Please describe.
Sanity's Asset CDN is awesome, but we already use Google Cloud Storage and Cloudflare as CDN for our website. It's frustrating that we're stuck with Sanity's CDN with no option of using our own over top for asset uploads.

Describe the solution you'd like

• Allow replacing the default Asset Upload destination with a custom one, e.g. GCP via the google-cloud/storage library
• Ideally that might look like providing a custom upload method in my sanity form config

Describe alternatives you've considered
Not sure what alternatives are out there

[devinhalladay]

More context: I built a custom plugin that adds a GCP bucket as an asset source

It works great but I want to also support uploading images to the bucket right from Sanity, and I also want Sanity to skip uploading images from this source to its own CDN, instead using the source URL from GCP