Add the Warning status to the commit API

This is in regards to this API feature: https://docs.gitlab.com/ee/api/commits.html#post-the-build-status-to-a-commit

The warning status, as described here: https://docs.gitlab.com/ee/ci/pipelines.html, is not available to the commit status API but it is an important and useful status as 'warning' is a valid build state to communicate back to the commit. For example, for our Nexus IQ integration with GitLab we communicate policy evaluation results for security vulnerabilities and licensing back to the GitLab commit and the only applicable/meaningful choices we currently have are 'success' and 'failed'. But, 'warning' is a very valid status to be able to communicate (as would also be shown on the pipeline mini-graph).

We would like to see the 'warning' status added to the commit status API please. This includes visualization of the warning state on the commit status.