LocalDatastoreHelper fails to start if used in an environment that cannot access the "gcd" GCS bucket

[aristide-n]

Is your feature request related to a problem? Please describe.
When attempting to use the Downloadable emulator, LocalDatastoreHelper downloads from a hardcoded URL to the file in the "gcd" GCS bucket:

java-datastore/google-cloud-datastore/src/main/java/com/google/cloud/datastore/testing/LocalDatastoreHelper.java

Line 75 in 1651b88

EMULATOR_URL = new URL("http://storage.googleapis.com/gcd/tools/" + FILENAME);

When used in an environment that cannot access the "gcd" GCS bucket, LocalDatastoreHelper fails to start (e.g. when running in a GCE VM protected by a VPC-SC perimeter).

Describe the solution you'd like
If I could provide the emulator file URL in an environment variable, I could copy the file from the "gcd" bucket into another bucket that is accessible from the restricted environment and use that URL.

Describe alternatives you've considered
N/A

[aristide-n]

I'm testing out a fix for this, I'll add a PR soon