feat: Support Narwhals

[hoxbro]

Still very much draft... A lot of the logic is currently copied/pasted from the PandasInterface.

import polars as pl
import pyarrow as pa

import holoviews as hv

hv.extension("bokeh", "matplotlib", "plotly")

data = {"a": [1, 2, 3], "b": [4, 5, 6]}
df_pandas = pd.DataFrame(data)
df_polars = pl.DataFrame(data)
table_pa = pa.table(data)

layout = hv.Curve(df_polars) + hv.Scatter(table_pa)
layout

[codecov]

Codecov Report

Attention: Patch coverage is 85.65022% with 96 lines in your changes missing coverage. Please review.

Project coverage is 88.78%. Comparing base (de3f00e) to head (f7781cc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/core/data/narwhals.py	83.55%	62 Missing ⚠️
...oloviews/tests/core/data/test_narwhalsinterface.py	88.81%	18 Missing ⚠️
holoviews/core/util/__init__.py	60.86%	9 Missing ⚠️
holoviews/operation/element.py	28.57%	5 Missing ⚠️
holoviews/util/transform.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6567      +/-   ##
==========================================
- Coverage   88.82%   88.78%   -0.04%     
==========================================
  Files         326      328       +2     
  Lines       69449    70060     +611     
==========================================
+ Hits        61685    62200     +515     
- Misses       7764     7860      +96     

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[hoxbro]

Would also make something like hv.Dataset(df_pandas).select(selection_expr=hv.dim('a')) fail