opt: unimplemented error for UDFs with mutations should reference an issue

[rytaft]

Describe the problem

UDFs don't currently support mutations, but the unimplemented error doesn't include a reference to an issue that tracks adding support.

To Reproduce

Run cockroach demo on master (or 23.1), and execute the following:

CREATE TABLE emp (name TEXT, salary INT);

CREATE FUNCTION clean_emp() RETURNS void AS '
    DELETE FROM emp
        WHERE salary < 0;
' LANGUAGE SQL;

This results in the error:

ERROR: unimplemented: DELETE usage inside a function definition
SQLSTATE: 0A000
HINT: You have attempted to use a feature that is not yet implemented.

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Expected behavior

The unimplemented error should reference a public GitHub issue that tracks adding support for DELETEs in UDFs. If such an issue doesn't already exist, we should open an issue to track this (and other unsupported statements such as UPDATE, CREATE TABLE, etc) and reference it in the unimplemented error. It will also allow community members to post if they particularly want the feature.

Jira issue: CRDB-26034