refactor: added more strict app segment config parsing (#70479)
This enhances the current parsing completed by Next.js of configuration
from app segments. Previously a collection of fragile checks was used to
parse the different segment configuration options which performed no
validation on the inputs.
This uses the `zod` library (which we already use internally) to perform
validation on the configuration exported. A followup PR #70480 will add
more verbose error logging around the validation errors.
