Parse Server session creation endpoint allows overwriting server-generated session fields

Parse Server is an open source backend that can be deployed to any infrastructure that can run Node.js. Prior to 9.6.0-alpha.17 and 8.6.42, an authenticated user can overwrite server-generated session fields (`sessionToken`, `expiresAt`, `createdWith`) when creating a session object via `POST /classes/_Session`. This allows bypassing the server's session expiration policy by setting an arbitrary far-future expiration date. It also allows setting a predictable session token value. Starting in version 9.6.0-alpha.17 and 8.6.42, the session creation endpoint filters out server-generated fields from user-supplied data, preventing them from being overwritten. As a workaround, add a `beforeSave` trigger on the `_Session` class to validate and reject or strip any user-supplied values for `sessionToken`, `expiresAt`, and `createdWith`.