fix(parquet): pass storage_options as **kwargs to fsspec.filesystem, fix mutable default args

[Tejas5405]

Problem

index_parquet_dataset() crashes with a TypeError when passing storage_options for S3 or any cloud filesystem:

TypeError: filesystem() takes 1 positional argument but 4 were given

The bug is in src/litdata/utilities/parquet.py line 153 — storage_options (a dict) is unpacked with * (which iterates keys as positional args) instead of ** (which passes key-value pairs as keyword args):

# Before ❌ — unpacks dict keys as positional args
self.fs = fsspec.filesystem(provider, *self.storage_options)

# After ✅ — passes key-value pairs as keyword args
self.fs = fsspec.filesystem(provider, **self.storage_options)

Additional fixes

Two places used a mutable default argument ({}) for storage_options, which is a well-known Python antipattern — a single dict is shared across all calls if not overridden:

# Before ❌
storage_options: dict | None = {}

# After ✅
storage_options: dict | None = None

Fixed in ParquetCloudIndexer.__init__ (line 81) and get_parquet_indexer_cls() (line 307). The existing None → {} guard in the base class handles the empty-dict default correctly.

Reproduction

import litdata as ld

ld.index_parquet_dataset(
    "s3://bucket/path/to/parquet/",
    storage_options={
        "key": "ACCESS_KEY",
        "secret": "SECRET_KEY",
        "endpoint_url": "https://s3.example.com",
    },
)
# Before fix: TypeError: filesystem() takes 1 positional argument but 4 were given
# After fix:  Works correctly

Closes #835