Route request based on namespace path

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For Cells 1.0, we need the following paths to be routable:

1. CDot related paths: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/8761#note_1940195643. Likely will change to internal paths.
   1. Addition, POST /api/v4/internal/gitlab_subscriptions/namespaces/:id/provision
2. Registry
3. Certain SAML / Omniauth callbacks #443478

/cc @jhyson @ayufan @bhrai @hswimelar @sxuereb @bmarjanovic