Allow --destination_dialect=bigquery in convert_file

[Chessing234]

Bug

The convert_file subcommand of mimic_utils rejects
--destination_dialect bigquery at argparse time, even though the
pipeline underneath supports bigquery as a destination. The
convert_folder subcommand accepts it.

$ python -m mimic_utils convert_file in.sql out.sql --destination_dialect bigquery
usage: python -m mimic_utils convert_file ...
python -m mimic_utils convert_file: error: argument --destination_dialect:
    invalid choice: 'bigquery' (choose from 'postgres', 'duckdb')

Root cause

In src/mimic_utils/__main__.py:

file_parser.add_argument("--destination_dialect",
    choices=["postgres", "duckdb"], default='postgres', ...)
...
folder_parser.add_argument("--destination_dialect",
    choices=["bigquery", "postgres", "duckdb"], default="postgres", ...)

file_parser is missing "bigquery" from its choices. Both subparsers
ultimately dispatch into transpile_file / transpile_folder, which
both call transpile_query(..., destination_dialect=...). That function
validates the dialect via _FUNCTION_MAPPING, which contains
{'bigquery': {}, 'postgres': {...}, 'duckdb': {...}} — so bigquery is
a supported destination; the CLI choice list is the only thing blocking
it.

Why the fix is correct

• The two subcommands call the same transpilation code; their
  destination choices should match.
• The underlying transpile_query / _FUNCTION_MAPPING already
  accepts bigquery as a destination dialect.
• Adds exactly one value to the choices list; default, help text, and
  dispatch remain unchanged.

Change

src/mimic_utils/__main__.py: add "bigquery" to the file_parser
--destination_dialect choices.