Implement Status checks in Branch Rules
### Description
As part of the MVC https://gitlab.com/groups/gitlab-org/-/epics/8075 for editing branch rules, we need to be able to add/remove status checks.
### Scope
To be updated by engineering
### Implementation plan
- Create `createExternalStatusCheck` mutation
- Create `editExternalStatusCheck` mutation
- Create `deleteExternalStatusCheck` mutation
### Mockups & behaviour
<details>
<summary>Details</summary>
[Figma](https://www.figma.com/file/eq2nf3B8MhB9xKrC0VzbPf/Repository?type=design&node-id=916%3A9220&mode=design&t=zrP2cLygKOMQqcde-1)
- Click "Add status check", display "Add status check" modal
- When "Edit" clicked, display "Update status check" modal
- When "Delete" icon button clicked, display delete modal (same like current)
- title: Remove status check?
- body: You are about to remove the **{name}** status check.
- destructive primary: Remove status check
- secondary: Cancel
- When change made and saved, display toast "Status check updated"
- When status check removed, display toast "Status check removed"
| Description | Mockup |
| ------ | ------ |
| Default view |  |
| Add status check |  |
| Update status check |  |
| Delete status check |  |
</details>
epic