Merge sorted not implemented for structs

[coastalwhite]

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

import polars as pl
from polars.testing import assert_series_equal

s = pl.Series('a', [{ 'x': 1 }], pl.Struct({ 'x': pl.Int32 }))

df = pl.DataFrame([s.sort()])

merge_sorted = df.lazy().merge_sorted(df.lazy(), 'a').collect().get_column('a')

Log output

thread '<unnamed>' panicked at crates/polars-ops/src/frame/join/merge_sorted.rs:152:13:
not implemented for dtype Struct([Field { name: "x", dtype: Int64 }])
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/home/johndoe/Projects/polars/bug.py", line 8, in <module>
    merge_sorted = df.lazy().merge_sorted(df.lazy(), 'a').collect().get_column('a')
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/johndoe/Projects/polars/py-polars/polars/lazyframe/frame.py", line 2056, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: not implemented for dtype Struct([Field { name: "x", dtype: Int64 }])

Issue description

Expected behavior

Installed versions

Replace this line with the output of pl.show_versions(). Leave the backticks in place.

[coastalwhite]

This should probably be implemented by converting to the row encoding then doing the merge sorted and then decoding from there.

[lukemanley]

xref: #13485

[aditya0yadav]

hi lukemanley coastalwhite
i can work on this
i have the required skill for this

[JakubValtar]

Hey @aditya0yadav, are you working on this?

I implemented it yesterday and I was just preparing to send a PR, but wanted to check with you first, since I forgot to claim it.