clightning: add trustedcoin plugin

[neverupdate]

Adds the trustedcoin plugin to allow using clightning without bitcoind, and an option to disable the bcli plugin in clightning, so trustedcoin can be used instead.

I didn't remove the clightning dependency on the bitcoind service though, but that could also be done.

[MatthewCroughan]

I actually did this exact same thing a week ago and never up-streamed it. Our results are basically identical! Bravo.

Can you split the commit messages up like so?:

trustedcoin: add pkg
trustedcoin: add module
readme: reference trustedcoin source
clightning-plugins: add trustedcoin
clightning: add useBcli option to module

Since nix-bitcoin is a library like nixpkgs, it makes sense to make the commit messages granular like this,

[neverupdate]

Should be good now!

[MatthewCroughan]

I suppose the only thing lacking, is a test. Because this plugin disables the bcli and doesn't require the bitcoin daemon in how it's recommended to be used, it is quite a tricky one to test that mode, without modifying the testing infrastructure.

[erikarvstedt]

Does it ever make sense to use trustedcoin with only bcli disabled?
With this setting, trustedcoin uses bitcoind-rpc as its main data source and falls back to public explorers when bitcoind-rpc fails.
But bitcoind-rpc should never fail with nix-bitcoin. Or might this be relevant during the initial block downloading in bitcoind?

Wouldn't it be better to add an option clightning.useBitcoind to disable bitcoind altogether when using clightning with trustedcoin? (I'd be happy to help implementing this.)