[Dashboard] [Meta] Provide a dedicated image panel

[AlonaNadler]

As part of the dashboard redesign. Allow users to add an image as a panel in a dashboard.
Currently, the way to add an image to a dashboard is using Markdown. However that is very hidden capability, it is not intuitive - users need to use markdown to add an image and frequently being missed. In addition image in markdown can only be added using a URL and not upload an image

It would be good to have an explicit add image panel where users can upload and image or add an image URL as part of the dashboard toolbar we are planning.

Allowing users to add an image will help create more customized dashboards.

Dependencies

• File storage service

Tasks

Plugin & Services

• Create image saved object
  • id (keyword): unique saved object id
  • value (binary): base64 dataurl
• Create image embeddable factory
  • should be backed by image expression
  • should this live in the expression-image plugin?
• Move asset function to image expression plugin
  • Rewrite asset function to retrieve saved objects instead of retrieving assets stored on workpad
  • Do we need to implement Canvas asset migration simultaenously? Do we need to create a new savedImage function for retrieving image saved objects and deprecate asset later?

Design & UI

• Should allow uploading images, entering a link to a hosted image, or selecting from the existing stored images
• Create standalone image upload/editor
  • Can reuse image upload form from Canvas
  • create action for displaying form in modal?
  • this component should be used for inline edit
• Create UI for managing images
  • can reuse asset manager component from Canvas

Dashboard Integration

• Add as quick button on toolbar and editor menu option in Dashboard
• Add a button somewhere to manage images?

Canvas Integration

• Switch to storing workpad assets as image saved objects instead of persisting assets to the workpad saved object
• Migrate assets stored on existing workpads and write them as image saved objects

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

+1 from a customer who'd like to have image upload support in dashboard. IT administrator was checking before spending the energy to host images somewhere in their infrastructure / network.

[cqliu1]

Closed with #146421.