feat: Provide way to hide dropdown programmatically

There is no way to programmatically close the gl-dropdown. I had encountered this issue when I was working on gitlab#29513 (closed). The mocks provided there show a 'cancel' button that is expected the close the dropdown.

Bootstrap has a way of hiding the dropdown as shown here https://bootstrap-vue.js.org/docs/components/dropdown/#optionally-scoped-default-slot. This needs to be enabled for the gl-dropdown component.

To address this issue, this MR exposes a hide method.