How to get a gitlabClient to call e.g. createMergeRequest(...) from Declarative Jenkins Pipeline

### Describe your use-case which is not covered by existing documentation.

We would like to call[ `GitLabClient#createMergeRequest(...)`](https://github.com/jenkinsci/gitlab-plugin/blob/9d32eea43bed3aaf93da9cc3aea6d92c74cd8aa8/src/main/java/com/dabsquared/gitlabjenkins/gitlab/api/GitLabClient.java#L19) from our Jenkinsfile (Declarative Pipeline).

How can we get a current instance of the GitLabClient? I did not find any documentation on this.

Thanks for helping out.

### Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

https://github.com/jenkinsci/gitlab-plugin/blob/9d32eea43bed3aaf93da9cc3aea6d92c74cd8aa8/src/main/java/com/dabsquared/gitlabjenkins/gitlab/api/GitLabClient.java#L19