Add initial very limited support for correlated subqueries

[mfussenegger]

Summary of the changes / Why this improves CrateDB

Adds support for correlated scalar subqueries within the select list. See https://github.com/crate/crate/issues/12663

• Introduces a new OuterColumn symbol to wrap columns from an outer relation.
• Adds a isCorrelated property to SelectSymbol to distinguish correlated & non-correlated subqueries
• Adds a new CorrelatedJoin operator. It takes an input relation and a correlated subquery. For each row in the input row it executes the subquery, outputting a joined row. The execution happens via a new CorrelatedJoinProjection and CorrelatedJoinProjector.

(I kept some individual commits but they must be squashed when merging)

Checklist

• [x] Added an entry in CHANGES.txt for user facing changes
• [x] Updated documentation & sql_features table for user facing changes
• [x] Touched code is covered by tests
• [x] CLA is signed
• [x] This does not contain breaking changes, or if it does:
  • It is released within a major release
  • It is recorded in CHANGES.txt
  • It was marked as deprecated in an earlier release if possible
  • You've thought about the consequences and other components are adapted (E.g. AdminUI)