With the ability to right-click, we need to be able to allow developers to implement custom context menus. On desktop platforms, these may need to be platform-specific context menus (so that they can render outside of the window), on web they will need to integrate with the context menu APIs there, and on Android they will need to be rendered by Flutter (one can right click on ChromeOS in an Android app, for instance. iOS doesn't support mouse, so it's not needed there). As a first pass on desktop platforms, it may initially be acceptable if the menu is rendered by Flutter (and hence doesn't render outside of the window), but long term probably not, so the API should definitely take that into account (and not take Widgets for the items, but rather some form of data object).

Before this, #31946 will probably need to be implemented, in order to provide the actions that the menu will invoke.

See also #30976 and #31412