This issue is a subtask of part two "Full Embedded SQL Client" of the implementation plan mentioned in FLIP-24.
It should be possible to declare user-defined functions in the SQL client. For now, we limit the registration to classes that implement ScalarFunction, TableFunction, AggregateFunction. Functions that are implemented in SQL are not part of this issue.
I would suggest to introduce a functions top-level property. The declaration could look similar to: