In this commit, we add a new TLV record that's intended to be used as an
explicit channel commitment type for a new form of funding negotiation,
and later on a dynamic commitment upgrade protocol. As defined, we have
3 channel types: base (the OG), tweakless, and anchors w/ zero fee
HTLCs. We omit the original variant of anchors as it was never truly
deployed from the PoV of lnd.
