Rank supports expressions

[czpmango]

Feature Description
In many scenarios, users will not (or cannot) specify the rank explicitly.
For example, the rank of bank transfer is set to the timestamp at that time, which needs to be generated by the now() function instead of manually specifying it.

Describe the solution you'd like
Rank supports expressions.

Additional context
Another solution to consider is to allow multiple insertion of the same type of edges and use attributes to distinguish them. In this case, the encoding of the underlying kv needs to be redesigned or a concept like rank need be embedded in the implementation layer without exposing it to the user.