[C++] Allow converting strings to dates without using datetimes as an intermediate step

### Describe the enhancement requested

```
import pyarrow as pa, pyarrow.compute as C
x = pyarrow.array(['2008-01-01', '2008-01-02', '2008-01-03'])
```

This works fine:

```
>>> C.strptime(x, format = '%Y-%m-%d', unit = 's').cast('date32[day]')

[
2008-01-01,
2008-01-02,
2008-01-03
]
```

But this isn't allowed:

```
>>> C.strptime(x, format = '%Y-%m-%d', unit = 'day')
Traceback (most recent call last):
File "pyarrow/_compute.pyx", line 1515, in pyarrow._compute._StrptimeOptions._set_options
KeyError: 'day'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "", line 1, in
File "/home/arferk01/.local/lib/python3.10/site-packages/pyarrow/compute.py", line 251, in wrapper
options = _handle_options(func_name, options_class, options,
File "/home/arferk01/.local/lib/python3.10/site-packages/pyarrow/compute.py", line 214, in _handle_options
return options_class(*args, **kwargs)
File "pyarrow/_compute.pyx", line 1538, in pyarrow._compute.StrptimeOptions.__init__
File "pyarrow/_compute.pyx", line 1519, in pyarrow._compute._StrptimeOptions._set_options
File "pyarrow/_compute.pyx", line 619, in pyarrow._compute._raise_invalid_function_option
ValueError: "day" is not a valid time unit
```

It seems wasteful to have to compute a full seconds-level POSIX timestamp when the only thing you have and that you want is the date.

Related: #31254.

### Component(s)

Python

Start with the Python option handling in pyarrow/_compute.pyx, especially StrptimeOptions and the C.strptime entry point shown in the report. Trace how time units are validated and locate the existing strptime tests. Done means date-valued parsing accepts the day unit directly and preserves the expected date results without requiring an intermediate datetime.

Written by the indexing model from the issue text.