feat: support cummulative window

[lmatz]

Is your feature request related to a problem? Please describe.

https://nightlies.apache.org/flink/flink-docs-master/docs/dev/table/sql/queries/window-agg/
Flink has this keyword.

RW can express it but we don't have a dedicated keyword and syntax sugar for it.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

requested by POC

[github-actions]

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