We filter based on the environment variable `CLN_PLUGIN_LOG`,
defaulting to `info` as that is not as noisy as `debug` or `trace`, at
least libraries will not spam us too heavily.
Changelog-Added cln-plugin: The logs level from cln-plugins can be configured by the `CLN_PLUGIN_LOG` environment variable.
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).