feat(gcloud): add option to run firestore in datastore mode

[Zarux]

What does this PR do?

This PR adds an option to the gcloud module applicable only to RunFirestore that runs the emulator in Firestore in Datastore mode.

It also changes the base example to point to the non-depracated function in order to create a new example with datastore mode

Why is it important?

Datastore in GCP is now Firestore in Datastore mode, which means the datastore emulator behaves slightly different to what one would expect as it emulates the legacy datastore product.

In the gcloud cloud-sdk version 465.0.0 a parameter was added to run the firestore emulator in datastore mode which should behave more like the product in GCP.

How to test this PR

The simplest way to test this would be to run the firestore emulator with the gcloud.WithDatastoreMode() and connect using the datastore package.
There is an example test case added in this PR as well that does this.

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[mdelapenya]

Thanks for submitting this improving. I took a quick look and initially it LGTM.

I want to share that after #3008, I'm considering following the same pattern of one submodule per package, deprecating all the existing GCP containers. As contributor of this GCP module, does it reasonable to you?

[Zarux]

Thanks for submitting this improving. I took a quick look and initially it LGTM.

I want to share that after #3008, I'm considering following the same pattern of one submodule per package, deprecating all the existing GCP containers. As contributor of this GCP module, does it reasonable to you?

My view is quite limited as this is my first contribution to this module 😄 But at first glance it looks reasonable, yes 👍