Add type annotations for kr8s._async_utils

[jacobtomlinson]

More fixes towards #381.

After this there are 44 mypy errors.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.33%. Comparing base (87063fc) to head (a50dc2c).
Report is 119 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/_async_utils.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
+ Coverage   94.61%   95.33%   +0.71%     
==========================================
  Files          29       30       +1     
  Lines        3141     3645     +504     
==========================================
+ Hits         2972     3475     +503     
- Misses        169      170       +1     

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