It used to be base64, which is not compatible with verifychanbackup,
expecting hex.

It is used for sweeping time-locked outputs as well as non time-locked outputs.

This pure function creates signed commit transaction, using various
inputs passed as struct TaprootSignedCommitTxInputs and a signer.

This is needed to be able to store the inputs without a signature
in SCB and sign the transaction in chantools scbforceclose.

See https://github.com/lightningnetwork/lnd/pull/8183/files#r1423959791

Previous to this change taproot assets channels and simple taproot channels were
considered the same in the context of chanbackup package, since they stored the
same data. In the following commits we are adding the data needed to produce a
signed commitment transaction from a SCB file and in order to do that we need to
add more fields and a custom channel gets one additional field (TapscriptRoot)
compared to a simple taproot channel. So now we have to distinguish these kinds
of channels in chanbackup package.

See PR lightningnetwork#8183 for more details.

The field is optional. It stores inputs needed to produce signed commit tx using
chantools scbforceclose, which calls function GetSignedCommitTx. New backups
have this field filled if commit tx is available (for all cases except when DLP
is active). If a backup has this data, the field is filled from it, otherwise it
is kept empty.

Modified test function genRandomOpenChannelShell to cover new types of channels
(simple taproot channel and custom channel) and to cover combinations of bits.
Make sure that TapscriptRoot field is properly packed and unpacked.

This method inserts channel updates and waits for them to be processed. It will
be used to update channel.backup upon LND shutdown.

This is needed to keep channel.backup up-to-date if the node is stopped.

Also updated release-notes.