添加manhattan路由的padding设置

[Fnxxxxo]

功能描述

当前1.32.2版本下，manhattan路由在线端点处添加标签和工具等会因为step较小，使图形“悬空”效果不好。增加step又会影响拖动体验（反复跳动）

希望能添加功能，指定预留空间

期望解决方案

希望能参考orth的padding设置，出一个

{
  startPadding: number;
  endPadding: number;
}

这样的args配置。使manhattan保持原有对target和source的设置效果，又能留出padding空间

[x6-bot]

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