Add oauth2 like auth service

Signed-off-by: Asai Neko <sugar@sne.moe>

config/types.go

@@ -47,6 +47,7 @@ type email struct {
 type secrets struct {
 	JwtSecret       string `yaml:"jwt_secret"`
 	TurnstileSecret string `yaml:"turnstile_secret"`
+	ClientSecretKey string `yaml:"client_secret_key"`
 }
 
 type ttl struct {