Rename Caption struct to CaptionMetadata

The Caption object does not actually store any text lines for the
subtitles. Instead it stores the metadata needed to display and fetch
the actual captions from the YT timedtext API.

Therefore it may be wiser to rename the struct to be more reflective of
its current usage as well as the future usage once the current caption
retrival system is replaced via InnerTube's transcript API

src/invidious/videos/caption.cr

@@ -1,7 +1,7 @@
 require "json"
 
 module Invidious::Videos
-  struct Caption
+  struct CaptionMetadata
     property name : String
     property language_code : String
     property base_url : String
@@ -10,12 +10,12 @@ module Invidious::Videos
     end
 
     # Parse the JSON structure from Youtube
-    def self.from_yt_json(container : JSON::Any) : Array(Caption)
+    def self.from_yt_json(container : JSON::Any) : Array(CaptionMetadata)
       caption_tracks = container
         .dig?("playerCaptionsTracklistRenderer", "captionTracks")
         .try &.as_a
 
-      captions_list = [] of Caption
+      captions_list = [] of CaptionMetadata
       return captions_list if caption_tracks.nil?
 
       caption_tracks.each do |caption|
@@ -25,7 +25,7 @@ module Invidious::Videos
         language_code = caption["languageCode"].to_s
         base_url = caption["baseUrl"].to_s
 
-        captions_list << Caption.new(name, language_code, base_url)
+        captions_list << CaptionMetadata.new(name, language_code, base_url)
       end
 
       return captions_list