mean and median for pl.Time should return a time

[mcrumiller]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Issue Description

Part of #13599. mean and median either do not work, return null, or do not return the proper dtype, except in a groupby context.

from datetime import time
import polars as pl

s = pl.Series([time(hour=1), time(hour=2), time(hour=15)])
df = pl.DataFrame({
    "key": [1, 1, 1],
    "s": s
})

### Series

print(s.mean())
print(s.median())
print(s.dt.mean())    # errors
print(s.dt.median())  # errors

21600000000000.0
21600000000000.0
AttributeError: type object 'Time' has no attribute 'time_unit'

Frame

print(
    df.select(
        col("s").mean().alias("mean"),
        # col("s").median().alias("median")  ## errors
    )
)

shape: (1, 1)
┌──────┐
│ mean │
│ ---  │
│ time │
╞══════╡
│ null │
└──────┘

GroupBy

print(
    df.group_by("key").agg(
        col("s").mean().alias("mean"),
        col("s").median().alias("median")
    )
)

shape: (1, 3)
┌─────┬──────────┬──────────┐
│ key ┆ mean     ┆ median   │
│ --- ┆ ---      ┆ ---      │
│ i64 ┆ time     ┆ time     │
╞═════╪══════════╪══════════╡
│ 1   ┆ 06:00:00 ┆ 02:00:00 │
└─────┴──────────┴──────────┘

Installed versions

--------Version info---------
Polars:               0.20.7
Index type:           UInt32
Platform:             Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:               3.12.1 (main, Jan 31 2024, 09:51:46) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  0.9.0
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            0.15.2
fsspec:               2023.12.2
gevent:               23.9.1
hvplot:               0.9.2
matplotlib:           3.8.2
numpy:                1.26.4
openpyxl:             3.1.2
pandas:               2.2.0
pyarrow:              15.0.0
pydantic:             2.6.1
pyiceberg:            0.5.1
pyxlsb:               1.0.10
sqlalchemy:           2.0.25
xlsx2csv:             0.8.1
xlsxwriter:           3.1.9