Merge branch 'id-add-explain-code-google-ai' into 'master'

Add VertexAI backend to Explain Code feature

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125292



Merged-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Approved-by: Nicolas Dular <ndular@gitlab.com>
Reviewed-by: Igor Drozdov <idrozdov@gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Reviewed-by: Bojan Marjanovic <bmarjanovic@gitlab.com>
Co-authored-by: Igor Drozdov <idrozdov@gitlab.com>