Merge branch 'jivanvl-add-pipeline-subscriptions-resolver' into 'master'

Add pipeline subscriptions GraphQL resolvers

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137819



Merged-by: Allison Browne <abrowne@gitlab.com>
Approved-by: Allison Browne <abrowne@gitlab.com>
Reviewed-by: Allison Browne <abrowne@gitlab.com>
Co-authored-by: Jose Ivan Vargas <jvargas@gitlab.com>