[Feature]: add support for `contentBefore` to Dropdown/Combobox

Library

React Components / v9 (@fluentui/react-components)

Describe the feature that you would like added

The Input component currently supports adding additional elements with the contentBefore/contentAfter properties. In Dropdown/Combobox there is a property called expandIcon which does more or less the same thing as contentAfter but there is no way of adding additional content before.

For example this would be useful to add a "magnifying glass" icon to a Combobox with autocomplete to indicate that it's a searchbox.

MS Teams (which kind of works like a ComboBox but has no expandIcon):

Google:

Have you discussed this feature with our team

No response

Additional context

No response

Validations

• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start in the React Components / v9 implementation by comparing how Input handles contentBefore/contentAfter with how Dropdown and Combobox handle expandIcon. Add the corresponding before-content API and update the relevant component coverage so a leading element, such as a search icon, renders correctly without disrupting the existing expand icon behavior.