One of the goals of this change is to have trunnel API/ABI being more explicit
so we namespace them with "trn_*". Furthermore, we can now create
hs_cells.[ch] without having to confuse it with trunnel which used to be
"hs_cell_*" before that change.

Here are the perl line that were used for this rename:

  perl -i -pe 's/cell_extension/trn_cell_extension/g;' src/*/*.[ch]
  perl -i -pe 's/cell_extension/trn_cell_extension/g;' src/trunnel/hs/*.trunnel
  perl -i -pe 's/hs_cell_/trn_cell_/g;' src/*/*.[ch]
  perl -i -pe 's/hs_cell_/trn_cell_/g;' src/trunnel/hs/*.trunnel

  And then "./scripts/codegen/run_trunnel.sh" with trunnel commit id
  613fb1b98e58504e2b84ef56b1602b6380629043.

Fixes #21919

Signed-off-by: David Goulet <dgoulet@torproject.org>