Allow passing getKey method to route rules

[MiniDigger]

Describe the feature

I want to use a custom cache key for my route cache (coming from nuxt but open to implementing a nitro plugin).

Naive me thought I could just pass it like this in my nitro (well, nuxt) config:

export default defineNitroConfig({
  routeRules: {
    '/**': {
      cache: {
        swr: true,
        maxAge: 600,
        name: 'pagecache',
        getKey: (e: any) => {
          console.log('hello', e);
          return 'dum';
        },
      },
    },
  },
});

that doesn't work because nitro can't serialize my method. fair enough.

my next try is a lot more ugly, but I basically did this as a nitro plugin:

export default defineNitroPlugin((nitroApp) => {
  let h = handlers.find(h => h.route === "/**");
  let handler = h.lazy ? lazyEventHandler(h.handler) : h.handler;
  console.log("installing handler", h.route);

  nitroApp.router.use(h.route, cachedEventHandler(handler, {
        group: "nitro/routes",
        swr: true,
        maxAge: 600,
        name: "pagecache",
        getKey: (e: any) => {
          console.log('hello', e);
          return 'dum';
        },
      }
  ), h.method);
});

it doesn't use any api and depends on implementation detail (handlers and cachedEventHandler aren't in scope in my plugin but are in scope in the compiled node-server.mjs), but it would be acceptable to me.

sadly not even that works, because the getKey method in my options object is overriden here: https://github.com/unjs/nitro/blob/main/src/runtime/cache.ts#L120

I am close to commenting that out in the compiled code via sed, but I figured I would open a feature request here first :D

I can't think of a nice api to make my plugin code nicer right now, since I don't have the handler cause it gets created by nuxt, but I would already be happy if cache.ts could be adjusted to not override getKey methods from the outside.

open to other ideas tho, I am willing to help brainstorming (I am MiniDigger#3086 on discord if that helps) and even implement this (haven't touched nitro code before but seems straight forward), my company allows me to work/contribute on this problem on work time.

Additional information

• Would you be willing to help implement this feature?