Feature request: implement cookie authentication for eclair RPC

[Kixunil]

Eclair could be better secured if it could generate its own cookie file and use it for authentication, just like bitcoind can.

Note that this is different from #1436, which is about Eclair -> bitcoind connection, while this one is about Lapp -> Eclair connection.

[t-bast]

I'd rather use something like lightning/blips#28 instead of exposing the RPC to the internet. Someone started to work on #2040 and dropped it, which indicates that this isn't a priority for the moment. Let's re-discuss when someone is ready to work on this.