Add PipelineStatus for "canceling"

[jschneider]

The documentation does not show this state here (https://docs.gitlab.com/api/pipelines/), but I definitely get the value canceling for a pipeline status.

I created a break point in org.gitlab4j.api.models.PipelineStatus#forValue

This is the screenshot for the pipeline in Gitlab UI:

[jmini]

Yes the documentation of GitLab REST API is not complete... Feel free to create a PR to this project to add the missing status.

[jschneider]

#1248