Add a `required_vec` TLV deserialization type

Add a `required_vec` TLV deserialization type

Historically, we used `vec_type` for all TLV Vec reads/writes, but
it is asymmetric and thus somewhat confusing - on the write side it
always writes a TLV entry, even if there are zero elements. On the
read side, it happily accepts a missing TLV, providing a
zero-length vector.

In 85b573ddad70f3c5ee36e0992d587842af507a8d a new `optional_vec`
TLV format was added which was symmetric, but only supports
optional vecs. This adds the corresponding required form, always
writing a TLV and ensuring it is present.