Merge branch 'timo-environments-api-cluster-agent-0237792c' into 'master'

Allow to configure cluster agent in Environments REST API

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/164864



Merged-by: Eduardo Bonet <ebonet@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Eduardo Bonet <ebonet@gitlab.com>
Reviewed-by: Tiger Watson <twatson@gitlab.com>
Co-authored-by: Timo Furrer <tfurrer@gitlab.com>