The fields provided by `DataLossProtect` have been mandatory since
lightning/bolts@6656b70, regardless
of whether `option_dataloss_protect` or `option_remote_key` feature bits
are set.

We move the fields out of `DataLossProtect` to make encoding definitions
more succinct with `impl_writeable_msg!` and to reduce boilerplate.

This paves the way for completely removing `OptionalField` in subsequent
commits.

As pointed out in lightning/bolts@6656b70,
we can move the `shutdown_scriptpubkey` field into the TLV streams of
`OpenChannel` and `AcceptChannel` without affecting the resulting encoding.

We use `WithoutLength` encoding here to ensure that we do not encode a
length prefix along with `Script` as is normally the case.