Extensible toolbar for code blocks in chat UI

[JasonWeill]

Problem

#789 added a toolbar with four hard-coded buttons to the bottom of each code block returned from a model provider in the chat UI: insert above active cell, insert below active cell, replace active cell, and copy the code to the clipboard. These options cannot be customized or disabled.

Proposed Solution

Use the toolbar registry, or another affordance, to let users disable buttons in the code block toolbar, add their own buttons, change button positions, or disable the toolbar.