Experiment with type stubs for the sync API

[jacobtomlinson]

I spent some time today experimenting with type stubs for the sync API (see #491).

This PR adds type stubs for the sync objects API. Right now it only implements the list() method on each object.

I then confirmed in kr8s/tests/test_objects.py::test_sync_pod_list that mypy is happy with these annotations.

The next step it to make a script that autogenerates these stubs based on the source files.

cc @florianvazelle for visibility

[codecov]

Codecov Report

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

Project coverage is 95.16%. Comparing base (87063fc) to head (196f2d4).
Report is 123 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/objects.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
+ Coverage   94.61%   95.16%   +0.54%     
==========================================
  Files          29       30       +1     
  Lines        3141     3988     +847     
==========================================
+ Hits         2972     3795     +823     
- Misses        169      193      +24     

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