Add pagination support to `QuickPick` API

<!-- ⚠️⚠️ 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 RemoteHub extension has repository pickers and branch/tag pickers that could contain 1000s of entries. Right now we have a hardcoded limit of 100 results, and it would be great to have an `onDidScrollToEnd` event which we can listen to to smoothly fetch more results into the quickpick.