lnutil: add LogPubKey helper function

This captures a common pattern where we want to log a peer's public key
along side each logging statement.

lnutils/log.go

@@ -1,8 +1,11 @@
 package lnutils
 
 import (
+	"log/slog"
 	"strings"
 
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btclog/v2"
 	"github.com/davecgh/go-spew/spew"
 )
 
@@ -36,3 +39,14 @@ func NewSeparatorClosure() LogClosure {
 		return strings.Repeat("=", 80)
 	}
 }
+
+// LogPubKey returns a slog attribute for logging a public key in hex format.
+func LogPubKey(key string, pubKey *btcec.PublicKey) slog.Attr {
+	// Handle nil pubkey gracefully, although callers should ideally prevent
+	// this.
+	if pubKey == nil {
+		return btclog.Fmt(key, "<nil>")
+	}
+
+	return btclog.Hex6(key, pubKey.SerializeCompressed())
+}