[Canvas] Use "Add New" Embeddable flow like Dashboard

[crob611]

Canvas currently uses an Add Embeddable flow that only allows you to add Saved Objects backed Embeddables.

The flyout used by dashboard includes an "Add New" button that lets you build embeddable certain kinds of embeddables (ML Swimlane) on the fly. Canvas should switch to using this method so that Canvas can take advantage of these new kinds of embeddables that do not require a saved object.

[elasticmachine]

Pinging @elastic/kibana-canvas (Team:Canvas)

[ryankeairns]

Proposed updates to the flyout are related to the toolbar work. Explained here: #82906

[cqliu1]

We want to use the same panel toolbar from Dashboard (see #85663) in Canvas to handle creating new by-value/by-reference embeddable and adding existing by-reference embeddables.

In order to integrate the panel toolbar, Canvas would need to generically support both by-value and by-ref for any type of embeddable. This is potentially a breaking change and requires some research.

This panel toolbar expects an Embeddable Container to be passed into the component to handle adding embeddables, so it'll require turning the Canvas workpad in an embeddable container.

Q: Do we need to turn workpads into an Embeddable Container? Would this support native elements? Would this require turning Canvas elements into embeddables?

To persist by-value embeddables, we also need to store the embeddable input object as part of the workpad object and will result in changes to the Workpad schema.

[clintandrewhall]

@cqliu1 In the immediate term, I'm more interested in having a consistent UI than I am a consistent embeddables architecture. In other words, while we can do research on the by-value/by-reference change, the components that make up the toolbar need not make that distinction... they just need to look the same. We could propose a pattern:

A toolbar in a Presentation solution consists of a blue "create" button, then three to five "symbol-based, quick create" buttons, (e.g. filter controls, common elements, text/link/etc), then a "full list" menu, then "Add from Library", (if applicable)

We could implement this pattern in Canvas today using the same components... just not those with particular business logic.

[crob611]

Just some notes from our meeting of generally what we think needs to happen to get this done. I will break this up into smaller issues that we can knock out one by one.

Embeddables by value in Canvas project

- Embeddable renderer should handle rendering embeddables by value
  - The embeddable renderer currently only does "Embeddable.createFromSavedObject" but should just do a standard "create" if there is no Id in the input
  - This might need an embeddable container to properly render
  - Switch to expression handler handlers.event for custom events instead of the canvas custom handlers
  

- Unrelated Task: Canvas uses renderer handlers outside of what's defined by Expressions.  
  - We should switch to use the provided handlers.event to fire our own events


- Create raw "embeddable" function,
  - This function would just take "config" as an argument. This config will just be a big old json blob.
  - We stll have the issue of action inside of the embeddable altering the input and would need to alter the underlying expression. (Example, recentering a map)
  - Maybe the embeddable input is part of the "element" 
  - We need to make sure that the blob displays nicely into the expression editor. 
    -Could be "go to lens" type stuff/etc 
  - Need a way to easily kick into "edit this config" 
  - Migrate the existing "savedMap" etc functions to just use this embeddable function

Telemetry and Migrations and References
  - Just like dashboard
  - Well tested

Switch to New Menu to create "By Values"
  - Handle state transfer for inserting new/edits into the workpad
  

- Filtering embeddables
  - I think the endgame should be that we hide the expression editor for embeddables
  - Then there is some kind of interface for selecting what filters are applied to that embeddable

- Canvas Embeddable Panel