[BUG] Support string to datetime conversion in Polars engine without explicit format

It looks like our string to datetime utilities throws an error. This is a fairly common step while cleaning datasets, so it'd be nice to support it:

```python
import polars as pl
from functools import partial
from cudf_polars.callback import execute_with_cudf
import numpy as np

use_cudf = partial(execute_with_cudf, raise_on_fail=True)

ldf = pl.DataFrame({
"date": ['2015-09-11', '2017-02-08', '2015-08-01', '2019-03-16', '2015-05-15'],
"val": [1, 2, 3, 4, 5]
}).lazy()

print(ldf.with_columns(pl.col("date").str.to_datetime()).collect())
print(ldf.with_columns(pl.col("date").str.to_datetime()).collect(post_opt_callback=use_cudf))
shape: (5, 2)
┌─────────────────────┬─────┐
│ date ┆ val │
│ --- ┆ --- │
│ datetime[μs] ┆ i64 │
╞═════════════════════╪═════╡
│ 2015-09-11 00:00:00 ┆ 1 │
│ 2017-02-08 00:00:00 ┆ 2 │
│ 2015-08-01 00:00:00 ┆ 3 │
│ 2019-03-16 00:00:00 ┆ 4 │
│ 2015-05-15 00:00:00 ┆ 5 │
└─────────────────────┴─────┘
---------------------------------------------------------------------------
ComputeError Traceback (most recent call last)
Cell In[141], line 14
12 ldf.sink_parquet("test.parquet")
13 print(ldf.with_columns(pl.col("date").str.to_datetime()).collect())
---> 14 print(ldf.with_columns(pl.col("date").str.to_datetime()).collect(post_opt_callback=use_cudf))

File [/raid/nicholasb/miniconda3/envs/all_cuda-122_arch-x86_64/lib/python3.11/site-packages/polars/lazyframe/frame.py:1942](http://10.117.23.184:8882/lab/tree/raid/raid/nicholasb/miniconda3/envs/all_cuda-122_arch-x86_64/lib/python3.11/site-packages/polars/lazyframe/frame.py#line=1941), in LazyFrame.collect(self, type_coercion, predicate_pushdown, projection_pushdown, simplify_expression, slice_pushdown, comm_subplan_elim, comm_subexpr_elim, cluster_with_columns, no_optimization, streaming, background, _eager, **_kwargs)
1939 # Only for testing purposes atm.
1940 callback = _kwargs.get("post_opt_callback")
-> 1942 return wrap_df(ldf.collect(callback))

ComputeError: 'cuda' conversion failed: NotImplementedError: String function StringFunction.Strptime