Add custom buttons to the toolbar in a global admin

[ccapndave]

I would like to add a few custom buttons to my collection and global admin pages. For a collection it can be done using admin.components.BeforeList, but there doesn't seem to be any way to do this for a global.

I propose adding a customisation hook to insert a React component in the DOM just before the Save button. If it was also possible to add a hook for the collections list that would render to the right of Create new that would be nice too.

[jacobsfletch]

@ccapndave try components.elements.SaveButton on your global. You could technically import the default save button into your component, and then render whatever else you'd like, including other buttons. But I see your point, might be easier through a config in the long-run.

[jacobsfletch]

I'm now seeing that the documentation around this is not entirely accurate, so I've updated that here: #3966