Add support for secure (.cookie) auth

[moodmosaic]

By default, Bitcoin Core generates unique login credentials each time it restarts and puts them into a file readable only by the user that started Bitcoin Core, allowing any of that user's RPC clients with read access to the file to login automatically.

The auth file is named .cookie and it's auto-generated in the Bitcoin Core data directory. Using the auto-generated credentials from the .cookie file is the preferred RPC authentication method.

As a fallback, one can directly use manually-chosen rpcuser and rpcpassword configuration parameters, but he or she must ensure that those are strong and unique (and still don't use insecure networks).

Based on configuration settings, bitcoinlib should be able to use either the .cookie or the manually-chosen rpcuser and rpcpassword.