引力导图中的symbol能不能设置为自定义的图形，也就是设置成graphic类型

### Version
4.9.0

### Steps to reproduce
引力导图中的symbol能不能设置为自定义的图形，也就是设置成graphic类型，官网给的(value: Array|number, params: Object) => string 这个函数只能返回字符串，没法返回一个graphic类型

### What is expected?
引力导图中的symbol设置为自定义的图形graphic类型

### What is actually happening?
引力导图中的symbol不能设置为自定义的图形graphic类型

Start with the gravity graph symbol customization API described in the issue, especially the callback whose value and params arguments currently return a string. Trace how symbols are created and how graphic elements are represented. Done means a gravity-graph symbol can accept a custom graphic type, with documented behavior and coverage for the new API.

Written by the indexing model from the issue text.