Offer a way to avoid the waterfall because of nested layouts, `await parent()` to retrieve `locals`

[GauBen]

Describe the problem

Let's say I have a few layouts that fetch data using session credentials.

// hooks.server.ts
export const handle = ({event, resolve}) => {
  if (hasACertainCookie) {
    event.locals.session = getSessionFromCookie()
  }

  return resolve(event)
}

// Root +layout.server.ts
export const load = ({locals}) => locals; // Expose all locals to layouts and pages

// Many +layout.ts and +page.ts look like this
export const load = ({parent}) => {
  const {session} = await parent();
  return { thing: fetch(..., session) }
}

This will unfortunately create a waterfall, despite the only needed data are the one from local.session

Describe the proposed solution

Svelte-Kit < 1 had a session feature which allowed constant-time access to a bag of top props, and it can be used as inspiration

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response