[ContextMenu] Auto-add menu to closest shadow root instead of the body

[web-padawan]

Describe your motivation

Currently, ContextMenu uses OverlayAutoAddController to add itself to the body, see #7792.

This might be a problem for embedded apps, e.g. in docs Flow examples CSS is not applied correctly:

Describe the solution you'd like

Consider using the same approach as Popover to add ContextMenu next to its target, or closest shadow root.
This is potentially a behavior altering change, so maybe there could be an API to make this configurable.