Add data test annotations for datastore, firestore and spanner

**Is your feature request related to a problem? Please describe.**
Make easy to slice test using datastore, firestoee and spanner

**Describe the solution you'd like**
Looking into [testing-code-that-uses-cloud-spanner](https://github.com/googleapis/google-cloud-java/blob/master/TESTING.md#testing-code-that-uses-cloud-spanner) I think the project can provide the infrastructure with @DataSpannerTest annotation. There are similar annotation for neo4j, jdbc, jpa and so on in the spring-boot project

See this [draft](https://github.com/spring-attic/spring-cloud-gcp/pull/2416)

**Describe alternatives you've considered**

**Additional context**

Start by reading the linked TESTING.md section on testing code that uses Cloud Spanner and the referenced spring-cloud-gcp draft pull request. Compare the proposed annotation approach for datastore, Firestore, and Spanner with the existing Spring Boot annotations. Done means the project provides and documents usable data-test annotations for the requested datastores.

Written by the indexing model from the issue text.