Expose Firestore backend timeout for configuration

https://github.com/firebase/firebase-js-sdk/blob/3dd9bcf6e1283677d95d91e4b20c75b997c00499/packages/firestore/src/remote/online_state_tracker.ts#L37

As a developer, I would like to be able to specify how long my client should wait on a connection to the database before it fails and throws an error; so that I can control my customers' experience.