新增PathGroup机制，解决多个path多个refD设置比较困难的问题

@hanyuesoyeon 需要设置相对 path。

如果我图形比较复杂，设置的path节点比较多，那我需要定义很多的rect框作为参照物，这很不方便，而且图形比较复杂的情况下，我可能无法定位参照框的位置和大小，能不能引入一种pathgroup的机制

_Originally posted by @NewByVector in https://github.com/antvis/X6/issues/3398#issuecomment-1632423387_

Start by reviewing the original discussion linked in the issue and the current handling of relative paths and refD in X6. Clarify the proposed PathGroup API and how multiple path nodes should share references before implementation. Done should mean complex graphics can define multiple relative paths without manually positioning many rect reference boxes.

Written by the indexing model from the issue text.