Hello devs! I'm currently working on a `TranslatableTable` implementation, in which I also use a custom implementation of `TableScan` that implements `EnumerableRel#implement`.
For this logic, I need to pass Model Objects through generated code for processing using `Expressions#constant`, similar to what is verified by this test <https://github.com/apache/calcite/blob/e17098d47f3c31e4d90cc17e6e1da1175bf49ae4/linq4j/src/test/java/org/apache/calcite/linq4j/test/ExpressionTest.java#L865> . Yet, for this to work I need to define models with public fields only, due to the check depending on matching a constructor with `value.getClass().getFields()` here <https://github.com/apache/calcite/blob/e17098d47f3c31e4d90cc17e6e1da1175bf49ae4/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java#L300> and here <https://github.com/apache/calcite/blob/e17098d47f3c31e4d90cc17e6e1da1175bf49ae4/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java#L216>. This means we can't use e.g. java records, and we also cannot declare Models with private fields and public accessors. This can be fixed by using `value.getClass().getDeclaredFields()` instead. I've created a small test branch to verify it here <https://github.com/apache/calcite/compare/main...WegdanGhazi:calcite:wip-constant-expression-fix> . I wanted to ask if there might be other reasons why this behaviour shouldn't be permitted. Thank you for your time!
