Include auto_merge in the REST API response
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### Proposal
With https://gitlab.com/gitlab-org/gitlab/-/issues/521291, `auto_merge` has replaced `when_pipeline_succeeds`.
While `when_pipeline_succeeds` is included in the API response, `auto_merge` is not.
It would help having this attributed in the API response, so that users can know if the MR is set to be merged automatically.
Related: [Zendesk ticket #631544(internal link)](https://gitlab.zendesk.com/agent/tickets/631544).
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