Expose `deployment_tier` in the “Pipeline events” webhook

Release notes

In GitLab 13.10, we introduced the concept deployment_tier. In this release, we added `deployment_tier in the Pipeline events webhook so that user can automate their processes based on the deployment_tier information.

Proposal

The pipeline event webhook currently returns the environment object, but not environment_tier. Having this information is useful particularly if non-standard names are used.

This request is related to #333261 (closed)