Trampoline prev/next_htlcs serialization #838

In #825, we ran into the orphan rule where we couldn't implement LDK's serialization traits for Vec<T> (even though we own T, we don't own Vec<T>). The proposed solution is:

• Write legacy fields (from prev_htlcs/next_htlcs) in Bump rust-lightning to include trampoline changes #825, which is okay because we haven't turned on trampoline in LDK yet so we're only every expecting a single HTLC in / out
• Update macro in LDK to provide the option to use default_for_vec - see util: add default_value_vec for defaults without LengthReadable rust-lightning#4507
• Update LDK-Node to use the macro and properly write new fields - draft in Update rust-lightning to use default_value_vec #850

We only need to start writing the full vec for prev/next_htlcs when we turn trampoline on, which isn't happening in the imminent 0.3 tag of LDK so it's not critical that the upstream LDK changes land quickly (though ofc would be nice).