Add ability for guest overage confirmation to return a promise

What does this MR do and why?

On the Manage -> Members page, clicking on a role for any member shows the role details drawer. Changing the role in the drawer and saving it will run a guest overage check. If an overage will happen, a warning modal is shown to the user:

Role details drawer	Overage warning modal

Previously, the overage check component required a callback method for when the overage check is confirmed. To make the component easier to use, this MR adds the ability for it to return a promise as well.

How to set up and validate locally

1. Enable the show_role_details_in_drawer feature flag:

echo "Feature.enable(:show_role_details_in_drawer)" | rails c

2. The guest overage feature is still under development. To force the modal to show, apply this patch:

Index: ee/app/assets/javascripts/members/components/table/drawer/guest_overage_confirmation.vue
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/ee/app/assets/javascripts/members/components/table/drawer/guest_overage_confirmation.vue b/ee/app/assets/javascripts/members/components/table/drawer/guest_overage_confirmation.vue
--- a/ee/app/assets/javascripts/members/components/table/drawer/guest_overage_confirmation.vue	(revision 3eb977ffcdaf12e05caa9f63c3e7cd9fa8f62caf)
+++ b/ee/app/assets/javascripts/members/components/table/drawer/guest_overage_confirmation.vue	(date 1720435073165)
@@ -74,7 +74,7 @@
     // modal. Otherwise, act as if the overage warning was accepted and emit the confirm event.
     checkOverage() {
       return new Promise((resolve, reject) => {
-        if (this.shouldSkipConfirmationCheck) {
+        if (!this.shouldSkipConfirmationCheck) {
           this.emitConfirm();
           resolve(true);
           return;
@@ -97,7 +97,7 @@
             const { willIncreaseOverage, seatsInSubscription, newBillableUserCount } =
               response?.data?.group?.gitlabSubscriptionsPreviewBillableUserChange || {};
             // If the overage won't increase or if there's no subscription data, don't show the modal.
-            if (!willIncreaseOverage || isNil(seatsInSubscription) || isNil(newBillableUserCount)) {
+            if (willIncreaseOverage || isNil(seatsInSubscription) || isNil(newBillableUserCount)) {
               this.emitConfirm();
               resolve(true);
               return;

3. Follow this video walkthrough (with audio commentary) for validation steps:

2024-07-08_00-42-32

Related to #464104