`psbt::raw::Key`'s `type_value` field should be serialized as `VarInt`

Refer to how `<keytype>` is defined in https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#specification

Mentioned by @brunoerg here: https://x.com/brrrunog/status/1803462216525447583