Merge branch 'mw/ai/proxy-code-suggestions' into 'master'

Add /completions endpoint to proxy to model gateway

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125401



Merged-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Reviewed-by: Matthias Käppler <mkaeppler@gitlab.com>
Reviewed-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Co-authored-by: bmarjanovic <bmarjanovic@gitlab.com>
Co-authored-by: Max Woolf <mwoolf@gitlab.com>
Co-authored-by: Matthias Käppler <mkaeppler@gitlab.com>