Provide support for separate aria label for the customButton on quickpick component

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> The quick pick component has only one aria-label that gets associated with the input box. https://github.com/microsoft/vscode/blob/main/src/vs/base/parts/quickinput/browser/quickInput.ts#L440 The quickpick component also has a customButton which does not have custom aria-label. https://github.com/microsoft/vscode/blob/main/src/vs/base/parts/quickinput/browser/quickInput.ts#L469 It would be nice to have a separate aria-label for this button.