libavformat: Add generic disc_subtitle tag mapping for asf

The mapping is taken from Picard's [1] and taglib's [2] documentation.

The mapping in Microsoft's docs [3] maps `TSST` from ID3 to
`WM/SetSubTitle`. However, some taggers [4] [5] use `WM/SubTitle` instead.
I believe this to be an error, especially since the official docs say otherwise.

[1]: https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html#disc-subtitle
[2]: https://taglib.org/api/p_propertymapping.html
[3]: https://learn.microsoft.com/en-gb/windows/win32/wmformat/id3-tag-support
[4]: https://github.com/quodlibet/quodlibet/blob/db95540de69c7f7cda662fbad6d90eeca89611ce/quodlibet/formats/wma.py#L43
[5]: https://invent.kde.org/multimedia/kid3/-/blob/989fb5b8f98e20387c704e229f8c8b9cd00ae453/src/plugins/taglibmetadata/taglibasfsupport.cpp#L93

libavformat/asf.c

@@ -37,6 +37,9 @@ const AVMetadataConv ff_asf_metadata_conv[] = {
     { "WM/Language",             "language"         },
     { "WM/OriginalFilename",     "filename"         },
     { "WM/PartOfSet",            "disc"             },
+    /* SetSubTitle can be found in this mapping:
+     * https://learn.microsoft.com/en-gb/windows/win32/wmformat/id3-tag-support */
+    { "WM/SetSubTitle",          "disc_subtitle"    },
     { "WM/Publisher",            "publisher"        },
     { "WM/Tool",                 "encoder"          },
     { "WM/TrackNumber",          "track"            },

tests/ref/fate/generic-tags-remux-asf

@@ -1,5 +1,5 @@
-1470acbb2e928d916f58a725a8d2ecd0 *tests/data/fate/generic-tags-remux-asf.asf
-20594 tests/data/fate/generic-tags-remux-asf.asf
+424865418175e7b42e77d4482fed91f1 *tests/data/fate/generic-tags-remux-asf.asf
+20596 tests/data/fate/generic-tags-remux-asf.asf
 #extradata 0:        4, 0x00020001
 #tb 0: 1/1000
 #media_type 0: audio