mkvmuxer: Support BlockAdditionMapping

Support BlockAdditionMapping in mkvmuxer::Track. This is used
for defining the type of BlockAdditional element that could
accompany a track.

mkvinfo output of the generated test file (showing the new part):
```
...
|  + Block addition mapping at 249 size 23 data size 20
|   + Block addition ID value: 4 at 252 size 4 data size 1
|   + Block addition ID name: itut35 at 256 size 9 data size 6
|   + Block addition ID type: 4 (ITU-T T.35) at 265 size 4 data size 1
...
```

BUG=b/432059314

Change-Id: I5fd5b97de10bd2a2745acb6f18ec18c05e26a8ab

• common/webmids.h[diff]
• mkvmuxer/mkvmuxer.cc[diff]
• mkvmuxer/mkvmuxer.h[diff]
• testing/mkvmuxer_test.cc[diff]
• testing/testdata/tracks_with_block_addition_mapping.webm[Added - diff]