[Feature] Circular Sankey

### What problem does this feature solve?

Hi,

First I'll start by saying this is probably the best charting library i've seen yet.

I'm developing a SaaS application and I'm visualizing some flows using a sankey diagram, but in the case of the flows I'm trying to show (which are API traffic flows), there are some 'circular' links -- which means some of the flows can 'go back' to a previous endpoint in the API (or a node in the diagram).

As far as I know there is no way to show it using e-charts (I've actually haven't seen any library that can do this except a custom solution in D3, which I have absolutely no will to use since I tried D3 many times, and while it's very versatile, I had a very bad overall DX with them.

Anyway, if it fits in your vision of echarts, I think it'll be a great feature to add :)

### What does the proposed API look like?

I'm not completely sure how the code will look like but I can give you some links to working examples in D3, I'm guessing you can get some inspiration from there.

Here's a circular-sankey D3 addon: https://www.npmjs.com/package/d3-sankey-circular

And here is a working example: https://bl.ocks.org/tomshanley/6f3fcf68c0dbc401548733dd0c64e3c3