Add support fo eager Narwhals dataframes

[cakiki]

This is a first attempt to add support for narwhals eager dataframes.

I tried as best I could to copy the pandas path.

Closes #1199 and #1496

[codecov]

Codecov Report

❌ Patch coverage is 99.59459% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.94%. Comparing base (c40548b) to head (962cc79).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datashader/utils.py	91.66%	2 Missing ⚠️
datashader/tests/test_pandas.py	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1502      +/-   ##
==========================================
+ Coverage   88.47%   88.94%   +0.46%     
==========================================
  Files          96       98       +2     
  Lines       19461    20173     +712     
==========================================
+ Hits        17219    17942     +723     
+ Misses       2242     2231      -11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will degrade performance by 22.69%

❌ 1 regressed benchmark
✅ 51 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	test_layout[forceatlas2_layout]	49.8 ms	64.4 ms	-22.69%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing cakiki:narwhals-eager (962cc79) with main (2c797e4)}

[cakiki]

@hoxbro this is ready I think