Add type annotations for kr8s._config

[jacobtomlinson]

Working towards #381. Resolves typing issues in kr8s._config.

The number of mypy errors is down to 12 with this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.32%. Comparing base (87063fc) to head (b1362a4).
Report is 119 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/_config.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
+ Coverage   94.61%   95.32%   +0.70%     
==========================================
  Files          29       31       +2     
  Lines        3141     3656     +515     
==========================================
+ Hits         2972     3485     +513     
- Misses        169      171       +2     

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