PostgreSQL query translation layer

[seshWCS]

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[clickhouse-ci]

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[alexey-milovidov]

All four of these queries work in ClickHouse as well.
It means that the test does not show anything about the PostgreSQL query language.

[alexey-milovidov]

It is pointless to obtain the AST in JSON and then parse it back.
We probably could work on the tree representation directly to avoid this serialization/deserialization step.

[alexey-milovidov]

This represents, at best, 30% of what makes practical sense.
Unless you want to continue working on this project, the PR has a low chance of being merged.