build+config: add default handler constructor

Since most of our projects will use the same handler duo and apply the
config options the same way, let's make our lives easier and add a
default handler constructor.

build/handlers.go

@@ -0,0 +1,23 @@
+package build
+
+import (
+	"os"
+
+	"github.com/btcsuite/btclog/v2"
+)
+
+// NewDefaultLogHandlers returns the standard console logger and rotating log
+// writer handlers that we generally want to use. It also applies the various
+// config options to the loggers.
+func NewDefaultLogHandlers(cfg *LogConfig, rotator *RotatingLogWriter) (
+	btclog.Handler, btclog.Handler) {
+
+	consoleLogHandler := btclog.NewDefaultHandler(
+		os.Stdout, cfg.Console.HandlerOptions()...,
+	)
+	logFileHandler := btclog.NewDefaultHandler(
+		rotator, cfg.File.HandlerOptions()...,
+	)
+
+	return consoleLogHandler, logFileHandler
+}