databases/rubygem-google-cloud-datastore: Add rubygem-google-cloud-datastore 2.8.0

google-cloud-datastore is the official library for Google Cloud Datastore.

Google Cloud Datastore is a fully managed, schemaless database for storing
non-relational data. Cloud Datastore automatically scales with your users and
supports ACID transactions, high availability of reads and writes, strong
consistency for reads and ancestor queries, and eventual consistency for all
other queries.