Bring `signrawtransactionwithkey` up to date

Currently unimplemented.

`signrawtransaction` has been obsoleted in favor of `signrawtransactionwithwallet` and `signrawtransactionwithkey` on bitcoin core, since v0.18.0.

We should support `signrawtransactionwithkey` in **lbcd**, and implement `signrawtransactionwithwallet` in **lbcwallet**