求支持自定义函数

背景：我们的项目中用流程画图的方式在写业务逻辑，把逻辑落地成gengine的 rule运行
问题：某些业务场景较为复杂，生成的规则代码超过几万行、里面大量的重复代码（例如一个流程图很多节点都会汇聚到一个最终处理结果的节点，做一些计算工作然后返回给上层业务，造成了很多重复代码）
需求：希望gengine可以支持在rule里面可以自定义函数。

The issue names no files, tests, or entry points. Start by locating the rule parser and execution path, then inspect how rule functions are currently represented. Done should mean rules can define reusable custom functions and invoke them without repeating the same generated logic.

Written by the indexing model from the issue text.