fix: `normalize-asset-prefix` adding leading slash when URL `assetPrefix` is provided (#68518)
### Why?
URL validation of `normalizeAssetPrefix` was wrong as it checks if is
starting with `://`:
https://github.com/vercel/next.js/blob/37df9ab243690187725a236cbf2debbcaeb33cfa/packages/next/src/shared/lib/normalized-asset-prefix.ts#L4-L7
This resulted the value to be `/https://example.com` instead of
`example.com`.
x-ref:
https://github.com/vercel/next.js/pull/68518#discussion_r1705162752
### How?
As `normalizeAssetPrefix` function was added for `getSocketUrl`, it
removes the protocol of the URL.
But for reusability, this could be a friction to other callers.
Therefore we validate the URL and let the callers handle the URL (e.g.
protocol).