sql: investigate if there is a way to not redact built-in function names

In https://github.com/cockroachdb/cockroach/pull/110870, we redact all function names because we are required to redact UDF names, and before type-checking we may not be able to distinguish UDFs from built-in functions with the same name. This issue is to investigate whether there are other signals, e.g., in function properties, which could indicate that a function is a built-in vs a UDF.

There are also some builtin function keywords (e.g., `substring`, `trim`) that can never be used for UDFs. There should be a way to distinguish these so that they do not have to be redacted.

Jira issue: CRDB-31879