mean and median for pl.Duration should return a duration

[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 works in expression and groupby contexts but not Series contexts.

from datetime import timedelta
import polars as pl
s = pl.Series([timedelta(days=1), timedelta(days=2), timedelta(days=15)])
df = pl.DataFrame({
    "key": [1, 1, 1],
    "s": s
})

Series

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

518400000000.0
172800000000.0
1970-01-07 00:00:00  # yikes
1970-01-03 00:00:00

Frame

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

shape: (1, 2)
┌──────────────┬──────────────┐
│ mean         ┆ median       │
│ ---          ┆ ---          │
│ duration[μs] ┆ duration[μs] │
╞══════════════╪══════════════╡
│ 6d           ┆ 2d           │
└──────────────┴──────────────┘

Groupby

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

shape: (1, 3)
┌─────┬──────────────┬──────────────┐
│ key ┆ mean         ┆ median       │
│ --- ┆ ---          ┆ ---          │
│ i64 ┆ duration[μs] ┆ duration[μs] │
╞═════╪══════════════╪══════════════╡
│ 1   ┆ 6d           ┆ 2d           │
└─────┴──────────────┴──────────────┘

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