Add t35_identifier_length and move descr to hrsd

[y-guyon]

The following succeeds:

t35_tool inject \
  --source "json-manifest:TestData/t35_tool/test_manifest.json" \
  --method "dedicated-it35" \
  --t35-prefix "B500900001:SMPTE-ST2094-50" \
  "TestData/isobmff/01_simple.mp4" \
  "/tmp/test_out.mp4"
t35_tool extract \
  --method "dedicated-it35" \
  "/tmp/test_out.mp4" \
  "/tmp/"

test_out.mp4