feat(mobile): use cached asset info if unchanged instead of downloading all assets (#1017)

* feat(mobile): use cached asset info if unchanged instead of downloading all assets

This adds an HTTP ETag to the getAllAssets endpoint and client-side support in the app.
If locally cache content is identical to the content on the server, the potentially large list of all assets does not need to be downloaded.

* use ts import instead of require

mobile/lib/utils/openapi_extensions.dart

@@ -0,0 +1,53 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:http/http.dart';
+import 'package:openapi/api.dart';
+
+import 'tuple.dart';
+
+/// Extension methods to retrieve ETag together with the API call
+extension WithETag on AssetApi {
+  /// Get all AssetEntity belong to the user
+  ///
+  /// Parameters:
+  ///
+  /// * [String] eTag:
+  ///   ETag of data already cached on the client
+  Future<Pair<List<AssetResponseDto>, String?>?> getAllAssetsWithETag({
+    String? eTag,
+  }) async {
+    final response = await getAllAssetsWithHttpInfo(
+      ifNoneMatch: eTag,
+    );
+    if (response.statusCode >= HttpStatus.badRequest) {
+      throw ApiException(response.statusCode, await _decodeBodyBytes(response));
+    }
+    // When a remote server returns no body with a status of 204, we shall not decode it.
+    // At the time of writing this, `dart:convert` will throw an "Unexpected end of input"
+    // FormatException when trying to decode an empty string.
+    if (response.body.isNotEmpty &&
+        response.statusCode != HttpStatus.noContent) {
+      final responseBody = await _decodeBodyBytes(response);
+      final etag = response.headers[HttpHeaders.etagHeader];
+      final data = (await apiClient.deserializeAsync(
+              responseBody, 'List<AssetResponseDto>') as List)
+          .cast<AssetResponseDto>()
+          .toList();
+      return Pair(data, etag);
+    }
+    return null;
+  }
+}
+
+/// Returns the decoded body as UTF-8 if the given headers indicate an 'application/json'
+/// content type. Otherwise, returns the decoded body as decoded by dart:http package.
+Future<String> _decodeBodyBytes(Response response) async {
+  final contentType = response.headers['content-type'];
+  return contentType != null &&
+          contentType.toLowerCase().startsWith('application/json')
+      ? response.bodyBytes.isEmpty
+          ? ''
+          : utf8.decode(response.bodyBytes)
+      : response.body;
+}