use toString() instead of cast String or it will fail when using Spring configtree (#10980)

Resolves #10979
Co-authored-by: Bruno Oliveira da Silva <bruno@abstractj.com>

adapters/oidc/adapter-core/src/main/java/org/keycloak/adapters/authentication/ClientIdAndSecretCredentialsProvider.java

@@ -45,7 +45,7 @@ public class ClientIdAndSecretCredentialsProvider implements ClientCredentialsPr
 
     @Override
     public void init(KeycloakDeployment deployment, Object config) {
-        clientSecret = (String) config;
+        clientSecret = (config == null ? null : config.toString());
     }
 
     @Override