Trigger the modal that informs users of additional charges when a Guest user in an Ultimate group is promoted to a higher role

This issue is a follow up to [SaaS] Modal that informs users of additional c... (&7804 - closed)

Problem

For Ultimate subscriptions (where Guest users are free), the promotion of a Guest user to a higher role can cause the Seats owed numbers to increase and the group to incur additional costs. This is an edge case that was not accounted for in the original issue.

Proposal

For Ultimate subscriptions, the modal that informs users of additional charges should trigger if a Guest user is promoted to a higher role and it would cause the Seats owed number to increase.

Implementation plan

Step	Notes	Status
Add feature flag :show_overage_on_role_promotion	1 MR either frontend or backend	workflowcomplete
Add a Vue component that would be shown every time a user is being promoted from Guest to a higher role. Use it to the RoleDropdown only if the FF is on. We need to intercept event that is happening in RoleDropdown but it is not an EE component, we would need an EE wrapper above it with handleSelect/handleCancel listeners	Extracted to a separate #363498 (closed) frontend	workflowcomplete
Add a service that would check points that the subscription is Ultimate and enrolled in reconciliations Use already existing check_overage.js to calculate overage Use the GraphQL API to check that	Split to #391532 (closed), either frontend or backend	workflowcomplete
Rollout :show_overage_on_role_promotion	1 MR either frontend or backend	workflowblocked