Make the logger customisable

Add a new `--log-format` cli argument to change the format of the logs.
The possible values are
- raw (the default one for old-style commands)
- bar (the default one for new-style commands)
- bar-with-logs (equivalent to `--print-build-logs`)
- internal-json (the internal machine-readable json format)

src/libmain/loggers.hh

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "types.hh"
+
+namespace nix {
+
+enum class LogFormat {
+  raw,
+  internalJson,
+  bar,
+  barWithLogs,
+};
+
+void setLogFormat(const string &logFormatStr);
+void setLogFormat(const LogFormat &logFormat);
+
+void createDefaultLogger();
+
+}