Add `rebase_in_progress` to `ExpandedMergeRequestSchema`

[omer-rivlin-platinum]

When adding the includeRebaseInProgress flag to the MergeRequests.show function, the gitbeaker response doesn't contain the rebaseInProgress property.

As stated in the GitLab documentation:

You can poll the Get single MR endpoint with the include_rebase_in_progress parameter to check the status of the asynchronous request.

If the rebase operation is ongoing, the response includes the following:

{
  "rebase_in_progress": true,
  "merge_error": null
}