Correctly set plural name in get when looking up unknown resources

[jacobtomlinson]

Closes #460

Updates lookup_kind() to also return the plural form of the resource kind. Also updates get_kind() to pass that plural along to new_class() to ensure dynamic classes are created correctly.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.11%. Comparing base (87063fc) to head (7fc9a3b).
Report is 93 commits behind head on main.

Files	Patch %	Lines
kr8s/_api.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #464      +/-   ##
==========================================
+ Coverage   94.61%   95.11%   +0.49%     
==========================================
  Files          29       30       +1     
  Lines        3141     3889     +748     
==========================================
+ Hits         2972     3699     +727     
- Misses        169      190      +21     

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