Add support to trigger a pipeline

[sjadema]

With #307, support for the Pipeline Triggers API was added. GitLab's API docs are missing the description to trigger a pipeline (using a trigger). Documentation about this can be found on a different page in their docs.

Would be awesome if we could trigger a pipeline as well.

[GrahamCampbell]

I'd accept a PR to implement this, with tests.

[sjadema]

Sure, i already implemented it for the project i needed it for. Tomorrow i'll backport it to this repo.

[sjadema]

Released in https://github.com/GitLabPHP/Client/releases/tag/11.1.0