[Datastore] Create indexes automatically for test data in builds

Some of the acceptance tests & samples tests for `datastore` require indexes to be created beforehand, which are specified in `google-cloud-datastore/acceptance/data/index.yaml` and `google-cloud-datastore/samples/index.yaml`. As of now, these need to be manually run every-time we make a change to indexes (instructions [here](https://github.com/googleapis/google-cloud-ruby/blob/main/.github/CONTRIBUTING.md#datastore-indexes)).

We should automate this process in the build through a rake task or something similar.

Problem identified in #18401