Support writing and reading back index

[j-bennet]

Implement writing and reading back Dask dataframe index.

Right now, after reading back, index will become just another column:

from dask.datasets import timeseries
from dask_deltatable import read_delta_table
from dask_deltatable.write import to_deltalake


if __name__ == "__main__":
    ddf = timeseries("2023-07-10", "2023-07-12", freq="1H")
    print(f"\nOriginal:\n{ddf.head()}")
    ddf.index = ddf.index.astype("datetime64[us]")
    to_deltalake("t2_data", ddf).compute()
    ddf2 = read_delta_table("t2_data")
    print(f"\nRead back:\n{ddf2.head()}")

Output:

Original:
                         name    id         x         y
timestamp
2023-07-10 00:00:00  Patricia   989  0.748295  0.513837
2023-07-10 01:00:00     Jerry   993  0.487847 -0.493534
2023-07-10 02:00:00    Victor  1035  0.851038 -0.941464
2023-07-10 03:00:00     Sarah   966  0.878928 -0.087768
2023-07-10 04:00:00     Quinn   983  0.712967 -0.977796

Read back:
       name    id         x         y           timestamp
0  Patricia   989  0.748295  0.513837 2023-07-10 00:00:00
1     Jerry   993  0.487847 -0.493534 2023-07-10 01:00:00
2    Victor  1035  0.851038 -0.941464 2023-07-10 02:00:00
3     Sarah   966  0.878928 -0.087768 2023-07-10 03:00:00
4     Quinn   983  0.712967 -0.977796 2023-07-10 04:00:00

Original PR: #29.