feat(runway): deploy ai-gateway-custom

What does this merge request do and why?

This extra deployment will be used for Staging Ref environment to enable self-hosted AI features.

Related discussion: https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/merge_requests/190#note_2140390595

Related issue: Functional Testing: Configure self-hosted model... (gitlab-org/gitlab#497784 - closed)