Split `sign_justice_transaction` in two halves

Split `sign_justice_transaction` in two halves

To avoid caller data struct storing HTLC-related information when
a revokeable output is claimed on top of a commitment/second-stage
HTLC transactions, we split `keysinterface::sign_justice_transaction`
in two new halves `keysinterfaces::sign_justice_revoked_output` and
`keysinterfaces::sign_justice_revoked_htlc`.

Further, this split offers more flexibility to signer policy as a
commitment revokeable output might be of a value far more significant
than HTLC ones.