Extensibility for the docs/canvas toolbar

[jgolieb]

Is your feature request related to a problem? Please describe.

The utility of this seems self-evident. If it already existed, I would've used it in the following two ways:

1. Add back the "open in new tab" button, which was removed for some reason. At least until it gets put back officially. On the other hand, if there was some reason that the core team didn't want to support that button as an out of the box feature, implementing extensibility would enable users to add it on their own.
2. Add a button to change the background on a per-story basis in docs. I sometimes have individual stories that need to have a different background than the others, so being able to change this for an individual story would be useful.

Describe the solution you'd like

I'd like a way to add buttons to the docs/canvas toolbar. It would be great if this worked similarly to how we can add buttons to the main toolbar via globalTypes in preview.ts. Behind the scenes, the values would be accessible via the StoryContext, so we could use them in decorators, etc. Maybe this feature would manipulate values in parameters, since those are already story-level values.

Describe alternatives you've considered

No response

Are you able to assist to bring the feature to reality?

no

Additional context

No response