[tiktok] extract subtitles and all cover types (#8805)

* Make sure that `img_id`, `audio_id` and `cover_id` fields are always available.
    The values are set '' where they are not applicable.
    Having `img_id` is necessary for the default `archive_fmt`, the other fields are handled for consistency.
* Allow downloading more than one cover.
    The previous behavior is kept as-is, but setting the "covers" option to "all" now grabs all available covers.
* Add support for downloading subtitles
    Allows filtering subtitles by source type (ASR, MT) and language.
* Ensure archive uniqueness for covers and subtitles.
* Update the URL test pattern to include the `image` extension.
    Although Tiktok may serve the covers with jpeg content, the file ending can be `.image`.
    The test before 0c14b164 failed because the asserted URL did not match all cover types, but the now used pattern needs the mentioned file ending.
* Add support for "creator_caption" subtitles in "LC" format.
    These subtitles have the keys "Format" set to "creator_caption" and "Source" to "LC".
* Add "LC" (Local Captions) as a subtitle source type in the documentation
* Code deduplication and renaming subtitle metadata
    Changed the item type from singular `subtitle` to `subtitles`.
    Removed the wrong descriptor `cover` from the subtitles fallback title.
* Refactor subtitle filtering
    The filter is now prepared in `_init` to prevent parsing the same config parameter for every item.
    The `_extract_subtitles` function will still extract if either filter (source or language) matches.
* Generate a `file_id` for subtitles
    Subtitles have multiple fields that determine the unique file, so these are simply concatenated.
    This is similar to the cover types, only with more variations.
* Added tests for subtitles
* fix docs entries
* fix '"covers": "all"'
* simplify some code
* Fix fallback title for subtitles
    Added the missing "f" to the f-string and added "subtitle" to the title.
    The resulting title will look like "TikTok video subtitle #1234567"

docs/configuration.rst

@@ -5914,12 +5914,25 @@ Description
 extractor.tiktok.covers
 -----------------------
 Type
-    ``bool``
+    * ``bool``
+    * ``string``
 Default
     ``false``
 Description
     Download video covers.
 
+    ``true``
+        Download the first cover found in the following order:
+
+        * ``thumbnail``
+        * ``cover``
+        * ``originCover``
+        * ``dynamicCover``
+    ``false``
+        Do not download covers
+    ``"all"``
+        Download all available covers
+
 
 extractor.tiktok.photos
 -----------------------
@@ -5931,6 +5944,47 @@ Description
     Download photos.
 
 
+extractor.tiktok.subtitles
+--------------------------
+Type
+    * ``bool``
+    * ``string``
+Default
+    ``false``
+Example
+    * ``"all"``
+    * ``"ASR,MT,LC"``
+    * ``"ASR,eng-US"``
+Description
+    Download video subtitles.
+    The subtitles can be filtered by source or language.
+    The following source types can be filtered:
+
+    * ``ASR`` - Automatic Speech Recognition
+    * ``MT`` - Machine Translation
+    * ``LC`` - Local Captions / Creator Captions
+
+    If both source types and language codes are provided,
+    only subtitles matching both are downloaded.
+
+    ``true``
+        Download all subtitles tagged ``ASR``
+    ``false``
+        Do not download subtitles
+    ``"all"``
+        Download all available subtitles.
+    ``"ASR,MT,eng-US,cmn-Hans-CN"``
+        Download english and simplified chinese subtitles
+        that are either automatically recognized or machine translated.
+
+        The source types and languages can be listed in any order.
+Note
+    It is not possible to filter all subtitles of a specific source type,
+    while also filtering for additional languages of another source type.
+    (e.g. any ASR subtitle + fra-FR of any source type)
+    For this, refer to `extractor.*.image-filter`_.
+
+
 extractor.tiktok.videos
 -----------------------
 Type

docs/gallery-dl.conf

@@ -825,10 +825,11 @@
         },
         "tiktok":
         {
-            "audio" : true,
-            "covers": false,
-            "photos": true,
-            "videos": true,
+            "audio"    : true,
+            "covers"   : false,
+            "photos"   : true,
+            "subtitles": false,
+            "videos"   : true,
             "tiktok-range": "",
 
             "posts": {