Custom Buttons and Labels for QuickPicks

<!-- ⚠️⚠️ 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. --> It would be great to be able to add some custom buttons to our QuickPicks, especially for our Azure Extension wizards. Specifically, being able to have an "OK" button would be useful so that the user can confirm without clicking on a quick pick item. Here is an example of what I am referring to:  I think that the simpleFileDialog already leverages some unexposed properties to do this already. https://github.com/microsoft/vscode/blob/91b97cec911a954bcd83d2613bb6d7b6bd4023ad/src/vs/workbench/services/dialogs/browser/simpleFileDialog.ts#L280-L281 @isidorn