Request for ability to set rank level for node for dagre mode

[alyuev]

• G6 Version:
• Platform:
• Mini Showcase(like screenshots):
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It will be good to have ability to set rank level for node for dagre mode. Like it has visjs.
Look at https://visjs.github.io/vis-network/docs/network/nodes.html
and find there:

"level | Number | undefined | When using the hierarchical layout, the level determines where the node is going to be positioned."

I think "rank level" will be the same as "level" option in visjs.

The exapmple of defferent level here:
https://visjs.github.io/vis-network/examples/network/layout/hierarchicalLayoutUserdefined.html

Node 2 on the next level, not as 10, 7, 12.

[Yanyan-Wang]

The dagre algorithm in G6 depends on https://github.com/dagrejs/dagre, the ranks are calculated automatically and not assignable in this lib. Same issue in dagrejs/dagre: dagrejs/dagre#289

[Yanyan-Wang]

duplicated to #2245