Add kubectl-ng cordon/uncordon

[jacobtomlinson]

Implemented kubectl-ng cordon NODE and kubectl-ng uncordon NODE.

Also found a typing issue with the return type from APIObject.get(). Updated to use the Self type instead.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.17%. Comparing base (87063fc) to head (0a92d7a).
Report is 121 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/_objects.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #500      +/-   ##
==========================================
+ Coverage   94.61%   95.17%   +0.55%     
==========================================
  Files          29       30       +1     
  Lines        3141     3978     +837     
==========================================
+ Hits         2972     3786     +814     
- Misses        169      192      +23     

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