Migrate `chan_utils` `vec_type` TLVs to required/optional

Migrate `chan_utils` `vec_type` TLVs to required/optional

* `CommitmentTransaction::htlcs` has always been written since the
  struct was converted to TLVs in 66784e32fe454e9a5b2080b85fc4d881.
* `HolderCommitmentTransaction::counterparty_htlc_sigs` have always
  been written since the struct was converted to TLVs in
  c8bc1b6d3de45aeddc755d0875b3dce8d86f79c1.