Offer `rpccookiefile` option as an alternative authentication method to bitcoind

[Scalena]

Bitcoin Core offers authenticating the RPC connection via a cookie file using the rpccookiefile option instead of rpcuser and rpcpassword. Cookie file authentication is considered more secure than username/password authentication, see [https://github.com/Kixunil/security_writings/blob/master/cookie_files.md] for a discussion. In my estimation it would be an easy-to-implement flexibility and security improvement if CLN supported a bitcoin-rpccookiefile option as an authentication method.

[cdecker]

To be fair, the rpcuser, rpcconnect and rpcpassword command line arguments are just there for usability, and not strictly required. You can achieve the same thing by having a ~/.bitcoin/bitcoin.conf that the bitcoin-cli can read on its own and pick up the rpccookiefile from there. In fact the first step to debug problems with the bitcoin backend is always to see if bitcoin-cli can connect to bitcoind on its own, as that's used under the hood.

As such replicating all possible authentication options for bitcoin-cli is likely out of scope, but I see that many users are switching over to rpccookiefile which should work out of the box if bitcoin-cli and bitcoind use the same home directory, so it might be ok to add this last authentication option to the bcli plugin too :-)

[Scalena]

That sounds reasonable, but what about cases, where the bitcoin.conf file is not in the default location? Then at least the location of the configuration file has to be supplied through bcli somehow, which is also not possible currently AFAIK. Perhaps this would be a better solution to include all possible authentication methods?