Route request based on namespace path

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.
2. Registry
3. Certain SAML / Omniauth callbacks #443478

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