Closes #179548

Summary

This PR makes it so that React embeddables will now work in Canvas. It does so by doing two main things:

1. It ensures that the ReactEmbeddableRenderer is used in Canvas when an embeddable exists in the React embeddable registry - if it does not exist, it continues to use the legacy embeddable factory's render method.

   Since Canvas auto-applies all changes and doesn't have save functionality like Dashboard does, we must keep track of changes as they happen and update the expression to match the new Embeddable input - therefore, I had to add a onAnyStateChange callback to the ReactEmbeddableRenderer as a backdoor for Canvas. As a bonus to this, embeddables that previously didn't respect inline editing (such as the Image embeddable) will start to work once they are converted!

2. It adds a new trigger (ADD_CANVAS_ELEMENT_TRIGGER) specifically for registering an embeddable to the Canvas add panel menu. This trigger can be attached to the same action that the Dashboard ADD_PANEL_TRIGGER is attached to - this makes it super simple to add React embeddables to Canvas:

   uiActions.registerAction<EmbeddableApiContext>({
     id: ADD_EMBEDDABLE_ACTION_ID,
     isCompatible: async ({ embeddable }) => { ... },
     execute: async ({ embeddable }) => { ... },
     getDisplayName: () => { ... },
   });
   
   // register this action to the Dashboard add panel menu:
   uiActions.attachAction('ADD_PANEL_TRIGGER', ADD_EMBEDDABLE_ACTION_ID);  
   
   // register this action to the Canvas add panel menu:
   uiActions.attachAction('ADD_CANVAS_ELEMENT_TRIGGER', ADD_EMBEDDABLE_ACTION_ID);

As a small cleanup, I also replaced some inline embeddable expressions with embeddableInputToExpression - this is because I was originally missing | render at the end of my expression, and I didn't catch it because the expressions I was comparing it to were all declared inline. This should help keep things more consistent.

How to Test

I attached the ADD_EUI_MARKDOWN_ACTION_ID action to the new ADD_CANVAS_ELEMENT_TRIGGER, so the new EUI Markdown React embeddable should show up in the Canvas add panel menu. Ensure that this React embeddable can be added to a workpad, and make sure it responds to edits as expected:

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately