[SPARK-49692][PYTHON][CONNECT] Refine the string representation of literal date and datetime

[zhengruifeng]

What changes were proposed in this pull request?

Refine the string representation of literal date and datetime

Why are the changes needed?

1, we should not represent those literals with internal values;
2, the string representation should be consistent with PySpark Classic if possible (we cannot make sure the representations are always the same because we only hold an unresolved expression in connect, but we can try our best to do so)

Does this PR introduce any user-facing change?

yes

before:

In [3]: lit(datetime.date(2024, 7, 10))
Out[3]: Column<'19914'>

In [4]: lit(datetime.datetime(2024, 7, 10, 1, 2, 3, 456))
Out[4]: Column<'1720544523000456'>

after:

In [3]: lit(datetime.date(2024, 7, 10))
Out[3]: Column<'2024-07-10'>

In [4]: lit(datetime.datetime(2024, 7, 10, 1, 2, 3, 456))
Out[4]: Column<'2024-07-10 01:02:03.000456'>

How was this patch tested?

added tests

Was this patch authored or co-authored using generative AI tooling?

no

[zhengruifeng]

I haven't find a standard method to convert timedelta to a ISO string like the JVM side

[zhengruifeng]

thanks @xinrong-meng , merged to master