feat(python!): Do not reset seek position when scanning from file-like objects

[nameexhaustion]

Currently they get reset to the start. After this PR we will instead read from the existing seek position.

I.e. -

import polars as pl
import io

f = io.StringIO("""\
A|B|C|D
1,2,3,4
""")

headers = f.readline().strip().split("|")

print(pl.scan_csv(f, has_header=False, new_columns=headers).collect())
# shape: (1, 4)
# ┌─────┬─────┬─────┬─────┐
# │ A   ┆ B   ┆ C   ┆ D   │
# │ --- ┆ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╪═════╡
# │ 1   ┆ 2   ┆ 3   ┆ 4   │
# └─────┴─────┴─────┴─────┘

[github-actions]

The uncompressed lib size after this PR is 57.8590 MB.

[github-actions]

The uncompressed lib size after this PR is 58.0356 MB.

[github-actions]

The uncompressed lib size after this PR is 58.0356 MB.

[github-actions]

The uncompressed lib size after this PR is 58.0356 MB.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.36%. Comparing base (7d58365) to head (c0d5c2c).

Files with missing lines	Patch %	Lines
crates/polars-python/src/file.rs	92.30%	2 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           nxs/2.0-csv-dispatch   #28619      +/-   ##
========================================================
- Coverage                 80.54%   80.36%   -0.19%     
========================================================
  Files                      1868     1868              
  Lines                    262210   262231      +21     
  Branches                   3141     3141              
========================================================
- Hits                     211206   210747     -459     
- Misses                    50191    50671     +480     
  Partials                    813      813              

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