feat: add spanner_create_index example (googleapis/google-cloud-cpp-spanner#1020)
feat: add spanner_create_index example (googleapis/google-cloud-cpp-spanner#1020)

* feat: add spanner_create_index example

* address review comment

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp