Improved passing of multiple postprocessor-args

* Added `PP+exe:args` syntax
    If `PP+exe:args` is specifically given, only it used.
    Otherwise, `PP:args` and `exe:args` are combined.
    If none of the `PP`, `exe` or `PP+exe` args are given, `default` is used
    `Default` is purposely left undocumented since it exists only for backward compatibility

* Also added proper handling of args in `EmbedThumbnail`

Related: https://github.com/ytdl-org/youtube-dl/pull/27723

youtube_dlc/postprocessor/execafterdownload.py

@@ -11,12 +11,15 @@ from ..utils import (
 
 
 class ExecAfterDownloadPP(PostProcessor):
-    PP_NAME = 'Exec'
 
     def __init__(self, downloader, exec_cmd):
         super(ExecAfterDownloadPP, self).__init__(downloader)
         self.exec_cmd = exec_cmd
 
+    @classmethod
+    def pp_key(cls):
+        return 'Exec'
+
     def run(self, information):
         cmd = self.exec_cmd
         if '{}' not in cmd: