Confirming UDF aliases are serialized correctly

[edmondop]

Which issue does this PR close?

I had created https://github.com/apache/datafusion/issues/11595 which I had created when working on https://github.com/apache/datafusion/pull/11013

When I was using create_aggregate_expr, aliases for max_udaf wouldn't be serialized, as indicated in #11595 . However, using the AggregateExprBuilder solved the problem, so the issue was probably in create_aggregate_expr , which by the way has been removed from the codebase, so we need to update the contributors how-to https://github.com/apache/datafusion/blob/main/docs/source/contributor-guide/howtos.md?plain=1#L62