[java-datastore] Make LocalDatastoreHelper aware of --use-firestore-in-datastore-mode

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**Is your feature request related to a problem? Please describe.**
I am using Firestore in Datastore mode, yet I cannot correctly emulate it in unit tests since I cannot configure --use-firestore-in-datastore-mode

**Describe the solution you'd like**
LocalDataStoreHelper should have the possibility to hand over the --use-firestore-in-datastore-mode parameter to the emulator.

**Describe alternatives you've considered**
Using Testcontainers

**Additional context**
https://github.com/GoogleCloudPlatform/spring-cloud-gcp/issues/1406