Introduce ApiType enum for API selection

This commit adds a new enum, `ApiType`, to allow dynamic selection between different base URLs for API calls. The enum has two values: `agent` and `benchmark`, corresponding to different services.

The `ApiType` enum is designed to be passed as a parameter to the `RestApiUtility` methods, enabling the utility to decide which base URL to use for HTTP requests.

frontend/lib/models/benchmark_service/api_type.dart

@@ -0,0 +1,4 @@
+enum ApiType {
+  agent,
+  benchmark,
+}