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

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

Start with the issue's examples in hooks.server.ts, +layout.server.ts, and the nested +layout.ts/+page.ts load functions, focusing on how await parent() exposes locals and creates a waterfall. Define a way to access session data without waiting through nested layouts, and verify that the resulting loads avoid the described waterfall while preserving access to session.