Add `impersonate` to `YoutubeDL._format_fields`

Authored by: bashonly

yt_dlp/YoutubeDL.py

@@ -613,7 +613,7 @@ class YoutubeDL:
         'player_url', 'protocol', 'fragment_base_url', 'fragments', 'is_from_start', 'is_dash_periods', 'request_data',
         'preference', 'language', 'language_preference', 'quality', 'source_preference', 'cookies',
         'http_headers', 'stretched_ratio', 'no_resume', 'has_drm', 'extra_param_to_segment_url', 'extra_param_to_key_url',
-        'hls_aes', 'downloader_options', 'page_url', 'app', 'play_path', 'tc_url', 'flash_version',
+        'hls_aes', 'downloader_options', 'impersonate', 'page_url', 'app', 'play_path', 'tc_url', 'flash_version',
         'rtmp_live', 'rtmp_conn', 'rtmp_protocol', 'rtmp_real_time',
     }
     _deprecated_multivalue_fields = {