Support using temporary access token to login to spanner

[felixyao]

WHAT

In some cases, (int my project github actions ). it is not safe to export the service account key files.
and since the use case is kind of temporally , so it is easier to use the access-token

gcloud print-access-token

and in spanner client using WithTokenSource option
options = append(options, option.WithTokenSource(ts))

WHY

In my cases ,update schema is short lived behaviour , and mostly happened in git hub actions . right now I have to use the key file , which have the risk to leak . using an access token in this scenario is much safer . and easy to maintain

Thanks