lnwallet: add new structs to abstract over the asymmetric musig commi…

…tment state

In this commit, we add a series of abstractions that'll allow us to
easily do funding and also state updates for the new taproot channels. A
partial session is defined by the knowledge of a verification nonce.
Once the remote party sends a signature, we learn of their signing
nonce, and can then complete a session. By using a JIT nonce approach,
we ensure that the signer can generate their nonces randomly and also
at the very last step to avoid having to maintain state.

For our local nonces, we also have an option to use a counter based
nonce derived from the shachain instead of fully random nonces. This
allows us to not have to store ay additional state. Instead, when we
need to go to broadcast, we can just regenerate the nonce then use that
to broadcast.