Return serviceIdentity in the AuthInfo.GetExtra if it's not empty (#175)

Return the `AccessTokenClaims.Rest.ServiceIdentity` in
`AuthInfo.GetExtra()` if the service identity is specified/not empty.

Author: mgyongyosi

authn/auth_info.go

@@ -71,7 +71,13 @@ func (a *AuthInfo) GetExtra() map[string][]string {
 		// but this should be removed in the not-to-distant future
 		return map[string][]string{"id-token": {a.id.token}}
 	}
-	return map[string][]string{}
+
+	result := map[string][]string{}
+	if a.at.Rest.ServiceIdentity != "" {
+		result[ServiceIdentityKey] = []string{a.at.Rest.ServiceIdentity}
+	}
+
+	return result
 }
 
 func (a *AuthInfo) GetAudience() []string {

authn/const.go

@@ -6,4 +6,6 @@ const (
 
 	httpHeaderAccessToken = "X-Access-Token"
 	httpHeaderIDToken     = "X-Grafana-Id"
+
+	ServiceIdentityKey = "serviceIdentity"
 )