[cli]: Support auto-complete on functions and tables

[prrao87]

Currently, the CLI only supports auto-complete on keywords like LOAD and ATTACH. This is great, but as we are adding more and more functionality (esp. to the extension framework), it would greatly improve usability if we could add auto-complete to function names and table names, too.

Benefits

• User doesn't need to remember the names/spellings of existing function names (for e.g., the show_attached_databases() function in the database extensions, which is rather long to type and not immediately obvious)
• User doesn't need to remember table names within databases (esp. in large Postgres/DuckDB databases they connect to externally) - because we store the Postgres/DuckDB table schemas in the cache this should be accessible from the CLI
• Better overall DevEx in the CLI, due to fewer overall keystrokes when typing