Make dummy_tables_path optional for starting the ehrQL sandbox

[alarthast]

Fixes #2184.

• Make the sandbox command default to using example-data if dummy_tables_path is not provided.
• If example-data does not exist, ask to dump it in the current directory (rationale in the ticket).
• To preserve the existing syntax, dummy_tables_path is an optional positional argument, so update cli.py to accommodate this.
• Add a banner message at the beginning of the sandbox stating the location of the dummy tables used, now that this is not an explicit argument for starting the sandbox. This replaces the default InteractiveConsole banner message.

[alarthast]

Closed since we are likely going to be replacing the sandbox with marimo notebooks