Reduce storage costs for automatic captions and subtitles

[tcely]

Looking at the English entry for automatic captions:

'en': [{'ext': 'json3',                                                                   
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'srv1',                                                                    
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'srv2',                                                                    
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'srv3',                                                                    
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'ttml',                                                                    
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'srt',                                                                     
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'},                                                
{'ext': 'vtt',                                                                     
'name': 'English',                                                                
'impersonate': True,                                                              
'__yt_dlp_client': 'android_vr'}],

We might change it to be something like this instead:

'en': [{'name': 'English',
'exts': ['json3', 'srv1', 'srv2', 'srv3', 'ttml', 'srt', 'vtt'],
}],

Originally posted by @tcely in #1534 (review)