Emit build error for unknown cache kinds

[unstubbable]

When a "use cache" directive with a custom cache kind is used, e.g. "use cache: custom", a cache handler with the same name must be specified in the Next.js config:

/**
 * @type {import('next').NextConfig}
 */
const nextConfig = {
  experimental: {
    dynamicIO: true,
    cacheHandlers: {
      custom: require.resolve('path/to/custom/cache/handler'),
    },
  },
}

module.exports = nextConfig

If this is not the case, we emit a build error with an error message that explains this requirement.

When we'll get a docs page for this experimental config, we will add the usual "Read more: ..." hint as well.

[unstubbable]

• Emit build error for unknown cache kinds #72832 👈 (View in Graphite)
• Consolidate server actions transform errors into emit_error function #72811
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[ijjk]

Tests Passed

[bgw]

Here's my suggested rust changes: 10e487a

• Some stuff to reduce cloning
• Pull RcStr into next-custom-transforms and use it across the config + transform
• Use a BodyInfo named struct for get_body_info instead of returning a less-obvious tuple type (not this PR's fault, but this PR touches it a lot).

Otherwise the Rust code LGTM.