Support signing keys

[paweljakubas]

This PR enables to decompose ANY extended signing key *_xsk into signing key and chain code.

New cli command was added which operate as follows:

$ cat drep.xsk
drep_xsk1vpdsm49smzmdwhd4kjmm2mdyljjysm746rafjr7r8kgfanj849psw8pfm305g59wng0akw3qzppmfh6k5z7gx66h2vppu022m4eqaj26rh6d7en9tf9fu52hmysjzuacaxfmfya65h8jmddrclwf3kxl8snfs3eg

$ cardano-address key private --signing-key < drep.xsk
drep_sk1vpdsm49smzmdwhd4kjmm2mdyljjysm746rafjr7r8kgfanj849psw8pfm305g59wng0akw3qzppmfh6k5z7gx66h2vppu022m4eqajg5xmwma

$ cardano-address key private --signing-key --hex < drep.xsk
605b0dd4b0d8b6d75db5b4b7b56da4fca4486fd5d0fa990fc33d909ece47a943071c29dc5f4450ae9a1fdb3a201043b4df56a0bc836b5753021e3d4add720ec9

$ cardano-address key private --chain-code < drep.xsk
5a1df4df66655a4a9e5157d9212173b8e993b493baa5cf2db5a3c7dc98d8df3c

$ echo drep_xsk1vpdsm49smzmdwhd4kjmm2mdyljjysm746rafjr7r8kgfanj849psw8pfm305g59wng0akw3qzppmfh6k5z7gx66h2vppu022m4eqaj26rh6d7en9tf9fu52hmysjzuacaxfmfya65h8jmddrclwf3kxl8snfs3eg | cardano-address key inspect
{
    "chain_code": "5a1df4df66655a4a9e5157d9212173b8e993b493baa5cf2db5a3c7dc98d8df3c",
    "extended_key": "605b0dd4b0d8b6d75db5b4b7b56da4fca4486fd5d0fa990fc33d909ece47a943071c29dc5f4450ae9a1fdb3a201043b4df56a0bc836b5753021e3d4add720ec9",
    "key_type": "private"
}

Also test vectors from CIP-0105 were verified (https://github.com/cardano-foundation/CIPs/tree/master/CIP-0105)