feat(python): Expose top-level "has_header" param for read_excel and read_ods

[alexander-beedie]

Closes #16944 by exposing "has_header" as a generic top-level param that behaves consistently across all supported engines.

Ref: #17727
Ref: #17265

Same param name (and behaviour) as read_csv).
Some additional internal cleanups.

Example

Reading from a sheet/table that has no column headers:

import polars as pl

for engine in ("calamine", "openpyxl", "xlsx2csv"):
  df = pl.read_excel(
    source="data.xlsx", 
    has_header=False, 
    engine=engine,
  )
  # shape: (3, 3)
  # ┌──────────┬──────────┬──────────┐
  # │ column_1 ┆ column_2 ┆ column_3 │
  # │ ---      ┆ ---      ┆ ---      │
  # │ i64      ┆ str      ┆ f64      │
  # ╞══════════╪══════════╪══════════╡
  # │ 1        ┆ aaa      ┆ 0.5      │
  # │ 2        ┆ bbb      ┆ 0.0      │
  # │ 3        ┆ ccc      ┆ -1.0     │
  # └──────────┴──────────┴──────────┘

[codecov]

Codecov Report

Attention: Patch coverage is 72.88136% with 16 lines in your changes missing coverage. Please review.

Project coverage is 80.34%. Comparing base (3dda47e) to head (0d36914).

Files	Patch %	Lines
py-polars/polars/io/spreadsheet/functions.py	72.88%	10 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18078      +/-   ##
==========================================
- Coverage   80.37%   80.34%   -0.03%     
==========================================
  Files        1496     1496              
  Lines      197542   197567      +25     
  Branches     2820     2829       +9     
==========================================
- Hits       158771   158744      -27     
- Misses      38249    38296      +47     
- Partials      522      527       +5     

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[ritchie46]

A rebase away...

[alexander-beedie]

A rebase away...

Great, thanks - missed this earlier; rebased :))