consolidate prefetch utils & separate build util (#61789)
In preparation for the refactor of cache utils in subsequent PRs, this PR moves the one-off prefetch cache utils into a shared `prefetch-cache-utils` file, and also relocates the `buildCustomRoute` util into an isolated lib so that it can be imported into base-server (in a future PR).
This PR does not make any changes to these functions.
Closes NEXT-2394
