[EuiInMemoryTable] Support nested panels in the default actions

[darnautov]

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Currently, there isn't any way of configuring nested panels for default actions in EuiInMemoryTable. To avoid creating a custom actions component, it'd be easier to support a behaviour similar to the generic EuiContextMenu

1. If the action is primary and the button is clicked directly from the row - show the configured panel, aligning it with the button.
2. If the action is accessed from the popover (shown after clicking the ellipsis icon), the behaviour should be similar to the nested panels in the EuiContextMenu