Create preview API to request preview SVGs from Editors

Editors should provide a preview for the assets. For this we should create a preview API, Which would be:

• New messages (REQUEST_PREVIEW, RESPOND_PREVIEW)
• Editor method that should be generated based on a new annotation (GetPreview is the annotation)

This method should be called from the envelop side and the behavior will change according to the channel:

• DESKTOP will use the feature to generate the preview image for the process/dmn. Users also can generate previews;
• ONLINE EDITOR and CHROME: A button will allow users to download the process image
• VSCODE: A command will generate the asset preview (Ctrl+Shift+P) in the same folder where the asset is placed.

By default GetPreview methods will return null (undefined) content.