Using existing node automatically

[Kixunil]

Idea mentioned in https://github.com/janoside/btc-rpc-explorer/pull/279#issuecomment-808699687

TL;DR: scroll down to "The simplest client-side implementation..."

Currently I solve this in my repository by simply placing proper configuration files in standard location and then reading them from other apps. This is still sub-optimal, so I intend to use interface files. I hope to refactor it in the following months.

The general idea is that we would standardize these locations:

• /etc/interfaces/$INTERFACE/*
• $XDG_CONFIG_HOME/interfaces/$INTERFACE/* ($XDG_CONFIG_HOME defaults to ~/.config)

Where $INTERFACE refers to interface name. In case of Bitcoin, we would have these interfaces:

• bitcoin-rpc-public-$NETWORK - can only execute harmless calls
• bitcoin-rpc-full-$NETWORK - full access to bitcoind wallet and chain.

$NETWORK refers to mainnet|testnet|regtest|signet. We can have more permission granularity in the future.

There's obligatory _default file in the interfaces directory. This can be used in case the consumer doesn't want to mess with handling multiple services.

The simplest client-side implementation is to just try to read and parse the files in this order:

1. $XDG_CONFIG_HOME/interfaces/bitcoin-rpc-public-$NETWORK/_default
2. ~/.config/interfaces/bitcoin-rpc-public-$NETWORK/_default (if XDG_CONFIG_HOME is not set)
3. /etc/interfaces/bitcoin-rpc-public-$NETWORK/_default

When one of them succeeds, break the loop and use it.

The contents of the file should be:

_spec_vesion = "1.0"
_iface_name = "bitcoin-rpc-public-mainnet"
# This is not bitcoind version but RPC and spec version
_iface_version = "1.0"
_impl_name = "bitcoind"

port = 8332
# user:pasword,
auth_token = "public:public"
# Alternatively:
# cookie_file = "/var/run/bitcoin-mainnet/cookie"