Update the tlv to use the type referenced in the spec, which also makes marks the tlv as optional.
Changelog-EXPERIMENTAL: --experimental-splicing: if you have splicing enabled on a channel both nodes must upgrade in unison due to updating commitment_signeds splice_info tlv's type.
The new spec sends `batch_size` in `start_batch` and removes it from `commitment_signed` so we need to stop processing it in `commitment_signed`.
Since the tlv is now reduced to one element and that automagically turns it into a direct use TLV so we have to update the code everywhere it is referenced.
We add `start_batch` to match t-bast’s splicing spec and we add a new internal wire type `WIRE_PROTOCOL_BATCH_ELEMENT` using the type number 0
Changelog-Added: support for `start_batch`