feat(python): Add lazy support for pl.select

[alexander-beedie]

Minor extension to pl.select, adding "eager" parameter so it can also operate in lazy mode and produce LazyFrames.

(Slightly ironically this function is already found in ".../functions/lazy.py" 😄)

Example

import polars as pl

lf = pl.select(n=pl.int_range(10, 0, -1), eager=False)
lf.filter((pl.col("n") % 3) == 0).collect()
# shape: (3, 1)
# ┌─────┐
# │ n   │
# │ --- │
# │ i64 │
# ╞═════╡
# │ 9   │
# │ 6   │
# │ 3   │
# └─────┘

FILTER [([(col("n")) % (3)]) == (0)] FROM
   SELECT [dyn int: 10.int_range([dyn int: 0]).alias("n")] FROM
    DF []; PROJECT */0 COLUMNS; SELECTION: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.52%. Comparing base (1f104f9) to head (23b2fa4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20091      +/-   ##
==========================================
- Coverage   79.53%   79.52%   -0.01%     
==========================================
  Files        1563     1563              
  Lines      217124   217125       +1     
  Branches     2464     2464              
==========================================
- Hits       172680   172679       -1     
- Misses      43884    43886       +2     
  Partials      560      560              

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