[ENH] improve Polars' LazyFrame complete

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Ensure lazyframe is lazy for as long as possible - avoid collect

import polars as pl
import janitor.polars

df.shape
(40000, 5)

# dev 
In [39]: %%timeit
    ...: df.lazy().janitor.complete(
    ...:         "group",
    ...:         pl.struct("item_id", "item_name").alias("rar").unique().sort(),
    ...:         fill_value={"value1": 0, "value2": 99},
    ...:         explicit=False,
    ...:         sort=True,
    ...:     )
    ...: 
6.21 ms ± 175 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

In [40]: %%timeit
    ...: df.lazy().janitor.complete(
    ...:         "group",
    ...:         pl.struct("item_id", "item_name").alias("rar").unique().sort(),
    ...:         fill_value={"value1": 0, "value2": 99},
    ...:         explicit=False,
    ...:         sort=True,
    ...:     ).collect()
    ...: 
11.9 ms ± 35.2 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)


# this PR
In [37]: %%timeit
    ...: df.lazy().janitor.complete(
    ...:         "group",
    ...:         pl.struct("item_id", "item_name").alias("rar").unique().sort(),
    ...:         fill_value={"value1": 0, "value2": 99},
    ...:         explicit=False,
    ...:         sort=True,
    ...:     )
    ...: 
111 µs ± 753 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [38]: %%timeit
    ...: df.lazy().janitor.complete(
    ...:         "group",
    ...:         pl.struct("item_id", "item_name").alias("rar").unique().sort(),
    ...:         fill_value={"value1": 0, "value2": 99},
    ...:         explicit=False,
    ...:         sort=True,
    ...:     ).collect()
    ...: 
5.91 ms ± 56.9 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

This PR relates to #1352 .

[ericmjl]

🚀 Deployed on https://deploy-preview-1378--pyjanitor.netlify.app

[ericmjl]

This one is an easy merge. Thank you, @samukweku!