[#607] *: Use zap.Stringer() where possible

Semantic patch:
```
@@
var f expression
var t expression
var a expression
@@
 f(
    ...,
-    zap.String(t, a.String()),
+    zap.Stringer(t, a),
    ...,
)
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/tree/signature.go

@@ -89,7 +89,7 @@ func (s *Service) verifyClient(req message, cid cidSDK.ID, rawBearer []byte, op
 		if !basicACL.AllowedBearerRules(op) {
 			s.log.Debug(logs.TreeBearerPresentedButNotAllowedByACL,
 				zap.String("cid", cid.EncodeToString()),
-				zap.String("op", op.String()),
+				zap.Stringer("op", op),
 			)
 		} else {
 			useBearer = true