[Feature] Support custom TopN pre-aggregation rules

[wu-sheng]

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Description

Currently, we have pre-aggregation topN endpoint on the service page. Meanwhile, in the root page(e.g. layer=general), we have a layer-level topN based on attr0.
Right now, we can't do pre-aggregation as we have limited the topN rules defined in source(OAL) through @ScopeDefaultColumn.BanyanDB(groupByCondInTopN = true), which is hard-coded and only supports one rule.

Use case

In the current scenario, to speed up these two kinds(maybe in the future), we should support more rules in the bydb.yml for customization requirements. Different users could have different scale data, which sometimes requires more preaggregation to achieve CPU-speed-disk_volume tradeoff -> Less CPU cost, and faster query performance, to trade off more disk_volume as pre-aggregation.

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• Yes I am willing to submit a pull request on my own!

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[wu-sheng]

@ScopeDefaultColumn.BanyanDB(groupByCondInTopN = true) and BanyanDB.TopNAggregation(in generated codes) could be considered to be removed, and we could only add those topN from the file configurations. Of course, we need some naming mechanism for query.