sql: implement virtual index on crdb_internal.{statement, transaction}_statistics

[Azhng]

Currently crdb_internal.{statement, transaction}_statistics is implemented using generator for the virtual table. This causes us to issue queries that perform full table scan on system.{statement, transaction}_statistics.

Since we are mostly doing scan based on aggregated_ts time ranges, we should add support for virtual index. This can enable us to issue better queries and avoid full table scan.

[blathers-crl]

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[Azhng]

virtual index cannot solve this problem. The underlying issue is the predicate is not being pushed down into the virtual table. Opened #74806 instead.