Load Flutter runtime config from Vault

lib/shared/runtime_config/runtime_config.dart

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+export 'runtime_config_stub.dart'
+    if (dart.library.js_interop) 'runtime_config_web.dart';

lib/shared/runtime_config/runtime_config_stub.dart

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+String mapboxAccessToken() => '';
+
+String mapboxStyle() => '';

lib/shared/runtime_config/runtime_config_web.dart

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+import 'dart:js_interop';
+
+@JS('__MAPFLOW_CONFIG__')
+external _MapflowConfig? get _config;
+
+extension type _MapflowConfig(JSObject _) implements JSObject {
+  external JSString? get mapboxAccessToken;
+  external JSString? get mapboxStyle;
+}
+
+String mapboxAccessToken() => _config?.mapboxAccessToken?.toDart ?? '';
+
+String mapboxStyle() => _config?.mapboxStyle?.toDart ?? '';