DMD-991: add list schemas

packages/php-storage-driver-common/proto/credentials.proto

@@ -71,6 +71,7 @@ message GenericBackendCredentials {
     string rest_endpoint = 2; // PostgREST API URL (e.g., https://xxx.supabase.co)
     string api_key = 3; // Supabase API key for REST API authentication
     bool allow_insecure = 4; // Allow HTTP for local testing (default: false)
+    string management_api_token = 5; // OAuth access token for Management API
   }
 
   google.protobuf.Any meta = 5; // metadata specific for each backend

packages/php-storage-driver-common/proto/info.proto

@@ -120,3 +120,15 @@ message TableInfo {
     }
   }
 }
+
+/**
+ * Command to list available schemas for external bucket registration.
+ * Returns user-accessible schema names excluding internal schemas.
+ */
+message ListSchemasCommand {
+  // Empty - no parameters needed, credentials provide connection info
+}
+
+message ListSchemasResponse {
+  repeated string schemas = 1;  // List of user-accessible schema names
+}