Improvements for prepared statements

We've improved support for prepared statements in Materialize. While we generally recommend using indexed views and/or materialized views over prepared statements, various external tools rely on prepared statements. As such, we've made the following improvements for better integration:

• Expanded support: Support prepared statement parameters in the OFFSET and RETURNING clauses.
• Improved type handling: Arguments to EXECUTE statements are automatically assignment-cast as needed.
• Better error reporting: Return 42P02 undefined_parameter SQLSTATE when appropriate.