feat(search): enable deep search with path expressions

[patricebender]

support search via path expressions:

• deep search via associations
• include calculated elements if explicitly requested

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to make this work, a non-enumerable searchTerm is attached to the SELECT.search object during inital infer call. The columns which should be searchable must be calculated early because the references need to be considered while building the joinTree for the given query.

In cqn4sql the fully linked search refs are then transformed as a regular function.

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rework of https://github.com/cap-js/cds-dbs/pull/252