add active style when dropdown is opened

[Teleology]

Describe the feature that you would like added

currently, the dropdown doesn't expose the styles for the open state. I prefer it to have something like below so I can customize the UI when the dropdown is in a different state:

export interface IDropdownStyles {
    root: IStyle;
   rootPressed: IStyle;
   rootChecked: IStyle;
   rootFocused: IStyle;
}

I want to highlight the box but didn't find a field such as rootOpened, so I use CSS and directly override it.

What component or utility would this be added to

IDropdownStyles

Have you discussed this feature with our team, and if so, who

No

Additional context/screenshots

No

[gouttierre]

@Teleology - Thanks for filling this enhancement for the IDropdownStyles. If you have a suggestion or would like to contribute towards this - please share a PR with us! For now, I’ll mark this as a feature request and add it to our backlog for the Dropdown Comps, but we’re likely not going to get to it for a while as we’re currently focused on larger coherence work items and enhancements to our performance.

[JustSlone]

Linking to our epic issue for Dropdown - #6030

[msft-fluent-ui-bot]

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