Add squash commits option in push options for merge requests
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### Release notes
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### Problem to solve
Currently, it is not possible to enable squash commits for a merge request when creating an MR through CLI(git) because there doesn't appear to be a [push option](https://docs.gitlab.com/ee/user/project/push_options.html) available for the same.
This is important if the project has [Squash commit options](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html#squash-commits-options) set to `Require` and the customer/user rely on git CLI to push and create the Merge Request (ex: `git push -o merge_request.create -o merge_request.merge_when_pipeline_succeeds -o merge_request.target=main`). As of now, such a MR won't merge as `squash` attribute [here](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/merge_requests/merge_service.rb#L63-64) will always be false.
Additional information: [Create MR](https://docs.gitlab.com/ee/api/merge_requests.html#create-mr) API has a `squash` attribute that helps in this scenario if the MR were created through the API.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
### User experience goal
User should be able to configure `squash` attribute when creating an MR through git CLI.
Example: `git push -o merge_request.create -o merge_request.merge_when_pipeline_succeeds -o merge_request.target=main -o merge_request.squash=true`
### Proposal
Adding a push option `squash` to the existing list of [Push options for merge requests](https://docs.gitlab.com/ee/user/project/push_options.html) should help here.
### Permissions and Security
* [ ] Add expected impact to members with no access (0)
* [ ] Add expected impact to Guest (10) members
* [ ] Add expected impact to Reporter (20) members
* [x] Add expected impact to Developer (30) members
* [x] Add expected impact to Maintainer (40) members
* [x] Add expected impact to Owner (50) members
### Documentation
Already linked in appropriate places.
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