sql: introduce a function sql_fuzz

SQL fuzzing test requires an entrypoint for SQL engine and the most
suitable function for such purpose is sql_stmt_compile(). However, this
function is not in a public API, and we don't want to expose it as
public function. As a solution function sql_fuzz() is introduced, it is
defined under a macro FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION.

Needed for #4826

NO_CHANGELOG=internal
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NO_TEST=internal