Make defaultClientScopes publicly accessible (#3785) (#2308)

This patch makes the `defaultClientScopes` variable publicly accessible
to allow for code that depends on the provider to have the ability to
programmatically determine the default OAuth2 scopes used by the
provider, and to add to it if necessary.

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3785.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

google-beta/config.go

@@ -298,7 +298,7 @@ var StorageDefaultBasePath = "https://www.googleapis.com/storage/v1/"
 var TPUDefaultBasePath = "https://tpu.googleapis.com/v1/"
 var VPCAccessDefaultBasePath = "https://vpcaccess.googleapis.com/v1/"
 
-var defaultClientScopes = []string{
+var DefaultClientScopes = []string{
 	"https://www.googleapis.com/auth/compute",
 	"https://www.googleapis.com/auth/cloud-platform",
 	"https://www.googleapis.com/auth/cloud-identity",
@@ -309,7 +309,7 @@ var defaultClientScopes = []string{
 
 func (c *Config) LoadAndValidate(ctx context.Context) error {
 	if len(c.Scopes) == 0 {
-		c.Scopes = defaultClientScopes
+		c.Scopes = DefaultClientScopes
 	}
 
 	tokenSource, err := c.getTokenSource(c.Scopes)