cln-plugin: Implement logging facade adapter for cln plugins

We wrap emitted messages into a JSON-RPC notification envelope and
write them to stdout. We use an indirection over an mpsc channel in
order to avoid deadlocks if we emit logs while holding the writer lock
on stdout.

plugins/Cargo.toml

@@ -10,11 +10,11 @@ path = "examples/cln-plugin-startup.rs"
 [dependencies]
 anyhow = "1.0.51"
 bytes = "1.1.0"
-log = "0.4.14"
+log = { version = "0.4.14", features = ['std'] }
 serde = { version = "1.0.131", features = ["derive"] }
 serde_json = "1.0.72"
 tokio-util = { version = "0.6.9", features = ["codec"] }
-tokio = { version="1", features = ['io-std', 'rt'] }
+tokio = { version="1", features = ['io-std', 'rt', 'sync'] }
 tokio-stream = "*"
 futures = "0.3"
 cln-rpc = { path = "../cln-rpc" }