sql/cmd/roachtest: improve pg_regress RLS test compatibility

Modify the rowsecurity.sql test from pg_regress to cover additional
cases. This test makes heavy use of SET SESSION AUTHORIZATION, which
CRDB doesn't support, so it has been adjusted accordingly. Additionally,
the test relies on a user-defined function f_leak; modifications were
made to allow this function to be created and used within the test.

Some incompatibilities remain. For example, CRDB doesn't support COPY
FROM in the same way as Postgres, which affects the expected output of
certain queries. Nevertheless, these changes improve the test suite.

Closes #137122
Epic: CRDB-11724

Release note: none

Author: spilchen