Create composables for the MR Widget
## Summary
Refactor the MR Widget to include `composables` to reduce the complexity in the `ready_to_merge.vue` component
## Improvements
Ideally, we should reduce the current amount of business logic inside the `ready_to_merge.vue` component to multiple smaller components with clear responsibilities so the code is easier to navigate and maintain.
## Risks
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Please list features that can break because of this refactoring and how you intend to solve that.
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## Involved components
https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
## Optional: Intended side effects
The MR Widget should behave the same as the current implementation.
## Optional: Missing test coverage
issue