[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"

test/results/tiktok.py

@@ -6,12 +6,13 @@
 
 from gallery_dl.extractor import tiktok
 
-PATTERN = r"https://p1[69]-[^/?#.]+\.tiktokcdn[^/?#.]*\.com/[^/?#]+/\w+~.*\.jpe?g"
+PATTERN = r"https://p1[69]-[^/?#.]+\.tiktokcdn[^/?#.]*\.com/[^/?#]+/\w+~.*\.(jpe?g|image)"
 PATTERN_WITH_AUDIO = r"(?:" + PATTERN + r"|https://v\d+m?\.tiktokcdn[^/?#.]*\.com/[^?#]+\?[^/?#]+)"
 VIDEO_PATTERN = r"https://v1[69]-webapp-prime.tiktok.com/video/tos/[^?#]+\?[^/?#]+"
 OLD_VIDEO_PATTERN = r"https://www.tiktok.com/aweme/v1/play/\?[^/?#]+"
 COMBINED_VIDEO_PATTERN = r"(?:" + VIDEO_PATTERN + r")|(?:" + OLD_VIDEO_PATTERN + r")"
 USER_PATTERN = r"(https://www.tiktok.com/@([\w_.-]+)/video/(\d+)|" + PATTERN + r")"
+SUBTITLE_PATTERN = r"https://v1[69]-[^/?#.]+\.tiktokcdn[^/?#.]*\.com/[^/?#]+/.*"
 
 
 __tests__ = (
@@ -127,10 +128,22 @@ __tests__ = (
     "#url"      : "https://www.tiktok.com/@memezar/video/7449708266168274208",
     "#comment"  : "video post cover image",
     "#class"    : tiktok.TiktokPostExtractor,
-    "#pattern"  : r"https://p19-common-sign-useastred.tiktokcdn-eu.com/tos-useast2a-p-0037-euttp/o4rVzhI1bABhooAaEqtCAYGi6nijIsDib8NGfC~tplv-tiktokx-origin.image\?dr=10395&x-expires=\d+&x-signature=.+",
+    "#pattern"  : PATTERN,
+    "#count"    : 1,
     "#options"  : {"videos": False, "covers": True},
 
 
+},
+
+{
+    "#url"      : "https://www.tiktok.com/@memezar/video/7449708266168274208",
+    "#comment"  : "all video post cover images",
+    "#class"    : tiktok.TiktokPostExtractor,
+    "#pattern"  : PATTERN,
+    "#count"    : 3,
+    "#options"  : {"videos": False, "covers": "all"},
+
+
 },
 
 {
@@ -211,6 +224,44 @@ __tests__ = (
     "#options" : {"videos": "ytdl"},
 },
 
+{
+    "#url"     : "https://www.tiktok.com/@memezar/video/7588916452304997635",
+    "#comment" : "default subtitles",
+    "#class"   : tiktok.TiktokPostExtractor,
+    "#pattern" : SUBTITLE_PATTERN,
+    "#count"   : 1,
+    "#options" : {"videos": False, "covers": False, "subtitles": True}
+},
+
+{
+    "#url"     : "https://www.tiktok.com/@memezar/video/7588916452304997635",
+    "#comment" : "english subtitles",
+    "#class"   : tiktok.TiktokPostExtractor,
+    "#pattern" : SUBTITLE_PATTERN,
+    "#count"   : 1,
+    "#options" : {"videos": False, "covers": False, "subtitles": "eng-US"}
+},
+
+# This test is prone to break when more translation agents are added!
+{
+    "#url"     : "https://www.tiktok.com/@memezar/video/7588916452304997635",
+    "#comment" : "combined subtitle filter",
+    "#class"   : tiktok.TiktokPostExtractor,
+    "#pattern" : SUBTITLE_PATTERN,
+    "#count"   : 6,
+    "#options" : {"videos": False, "covers": False, "subtitles": "ASR,deu-DE"}
+},
+
+# This test is prone to break when new languages or more translation agents are added!
+{
+    "#url"     : "https://www.tiktok.com/@memezar/video/7588916452304997635",
+    "#comment" : "all subtitles",
+    "#class"   : tiktok.TiktokPostExtractor,
+    "#pattern" : SUBTITLE_PATTERN,
+    "#count"   : 64,
+    "#options" : {"videos": False, "covers": False, "subtitles": "all"}
+},
+
 {
     "#url"      : "https://vm.tiktok.com/ZGdh4WUhr/",
     "#comment"  : "vm.tiktok.com link: many photos",