[Feature] image block component: isUploading indicator during upload

[Laurin-W]

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

When the onUpload callback takes a long time (e.g. the connection is bad), the image won't appear until the upload is finished and there is no loading indicator. That can result in irritated users.

Solution

The component could show a loading indicator.
Optionally, the image could be shown already using an URL.createObjectURL which is replaced when the onUpload callback resolves.

Alternatives

Thanks for the amazing work on this project! 😄