Merge branch 'ck3g-add-streaming-to-code_completions-api-endpoint' into 'master'

Add `stream: true` to /code_suggestions/completions API

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/136553



Merged-by: Alessio Caiazza <acaiazza@gitlab.com>
Approved-by: Alessio Caiazza <acaiazza@gitlab.com>
Reviewed-by: Alessio Caiazza <acaiazza@gitlab.com>
Reviewed-by: Mikhail Mazurskiy <mmazurskiy@gitlab.com>
Co-authored-by: Mikhail Mazurskiy <mmazurskiy@gitlab.com>
Co-authored-by: Vitali Tatarintev <vtatarintev@gitlab.com>