Write_delta doesn't cast datetime columns to us precision

[ion-elgreco]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

df = pl.from_repr("""┌─────────────────┬───────────────────┬───────┬────────────────────────────────┐
│ source_actor_id ┆ source_channel_id ┆ ident ┆ timestamp                      │
│ ---             ┆ ---               ┆ ---   ┆ ---                            │
│ u32            ┆ i64               ┆ str   ┆ datetime[ns]      │
╞═════════════════╪═══════════════════╪═══════╪════════════════════════════════╡
│ 123456780       ┆ 9876543210        ┆ a:b:c ┆ 2023-03-25 10:56:59.663053  │
│ 803065983       ┆ 2055938745        ┆ x:y:z ┆ 2023-03-25 12:38:18.050545  │
└─────────────────┴───────────────────┴───────┴────────────────────────────────┘""")

df.write_delta('test', mode='append')
Exception: Schema error: Invalid data type for Delta Lake: Timestamp(Nanosecond, None)

Issue description

Delta primitive type timestamp only supports "us" precision.

Expected behavior

Polars to cast all timestamp types into 'us' precision for it to work with delta primitive types.

Installed versions

--------Version info---------
Polars:              0.18.9
Index type:          UInt32
Platform:            Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:              3.11.4 (main, Jul 16 2023, 15:13:18) [GCC 11.3.0]

----Optional dependencies----
adbc_driver_sqlite:  <not installed>
cloudpickle:         <not installed>
connectorx:          <not installed>
deltalake:           0.10.1
fsspec:              2023.6.0
matplotlib:          <not installed>
numpy:               1.25.1
pandas:              2.0.3
pyarrow:             12.0.1
pydantic:            <not installed>
sqlalchemy:          <not installed>
xlsx2csv:            <not installed>
xlsxwriter:          <not installed>

[ion-elgreco]

Actually issue is already tracked in delta-rs, I'll try to fix it there.

delta-io/delta-rs#1467

[ion-elgreco]

@stinodego Reopening it actually, I have given it some thought. Polars should not submit a schema override that contains dtypes that are incompatible with the delta primitive types.

[ion-elgreco]

@stinodego

Almost done with a working solution, just need to extend it to Structs as well. I