Ignore ReadOptions on getAll

[op]

This change ignores the optional ReadOptions in firestore.getAll. It also raises an exception if no argument were passed at all.

If you use the firebase-admin package and call getAll with only one argument; ReadOptions -- it will actually do the round-trip to the backend. If you have no arguments at all, it raises an exception and no promise is created. This behaviour is also mimicked.

The first argument is required and must be of type DocumentReference followed by any additional DocumentReference documents. If used, the optional ReadOptions must be the last argument.

References: https://cloud.google.com/nodejs/docs/reference/firestore/latest/Firestore#getAll

[dmurvihill]

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