fix(spanner): restore removed scopes (#3684)

diff --git a/spanner/client.go b/spanner/client.go
index cd02d23..fcba322 100644
--- a/spanner/client.go
+++ b/spanner/client.go
@@ -44,6 +44,14 @@
 	numChannels = 4
 )
 
+const (
+	// Scope is the scope for Cloud Spanner Data API.
+	Scope = "https://www.googleapis.com/auth/spanner.data"
+
+	// AdminScope is the scope for Cloud Spanner Admin APIs.
+	AdminScope = "https://www.googleapis.com/auth/spanner.admin"
+)
+
 var (
 	validDBPattern = regexp.MustCompile("^projects/(?P<project>[^/]+)/instances/(?P<instance>[^/]+)/databases/(?P<database>[^/]+)$")
 )