Support ReadOnly transaction

[apstndb]

Documentation:
Cloud Firestore Datastore mode
https://cloud.google.com/datastore/docs/concepts/transactions?hl=en#read-only_transactions
Cloud Datastore
https://cloud.google.com/datastore/docs/concepts/cloud-datastore-transactions?hl=en#uses_for_transactions

Library support(App Engine Datastore and Cloud Datastore):
https://pkg.go.dev/google.golang.org/appengine@v1.1.0/datastore#TransactionOptions
https://pkg.go.dev/cloud.google.com/go/datastore@v1.0.0#TransactionOption

[s-takehana]

Is there a workaround?