This commit adds signet support, which is a little bit silly/redundant for us :)
Was a surprisingly easy merge to handle, and hopefully in future Core is more
mindful of signed blocks when they are changing code architecture.
I had to change a couple lines of src/signet.cpp to add blank assets to the
CTxOuts and to find transaction input scriptWitnesses. No need to add any
other tx witness data (and the CAsset()s that I did add to make things compile
won't be used..) because signet will always have g_con_elementsmode off.
m_valid implies the block solution has been checked, which is not the
case. It only means the txs could be parsed. C++17 comes with
std::optional, so just use that instead.