[Bug] Unable to use custom claims in firestore security rule when querying from C++ SDK

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp)
* Firebase C++ SDK version: 13.0.0
* Problematic Firebase Component: Auth and Firestore
* Other Firebase Components in use: Functions, Storage
* Platform you are using the C++ SDK on: Mac
* Platform you are targeting: Desktop and iOS

### [REQUIRED] Please describe the issue here:

I get Missing or insufficient permissions when querying firestore collection that has a security rule that looks like:
```
match /subscriptions/{subscriptionId} {
allow read: if request.auth != null && request.auth.token.legacy_id == resource.data.user_id;
}
```

I am using the C++ SDK, which doesn't give direct access to the auth custom claims, but I am able to fetch custom claims via a Firebase function, and see the values correctly in the client after those are fetched.

If I write a security rule that depends on those claims, it fails. Does writing a rule that uses request.auth.token.legacy_id work when the request comes from the C++ SDK?

#### Steps to reproduce:

What's the issue repro rate? 100%

It appears that using a custom claim (say, a numeric secondary ID) and requiring that to match a field on a document doesn't work when the query is run from the C++ SDK.