Add PortForward.run_forever(), add docs examples and fix sync API bug

[jacobtomlinson]

Added a run_forever() method to the PortForward object to indefinitely block and serve the port forward.

from kr8s.objects import Pod

pod = Pod.get("my-pod")
pod.portforward(1234, local_port=5678).run_forever()

Also added some documentation examples which closes #217 and a couple of fixes to the way the loop is handles which closes #218.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a5b2b11) 95.06% compared to head (0085570) 94.99%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
kr8s/_portforward.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   95.06%   94.99%   -0.07%     
==========================================
  Files          26       26              
  Lines        2575     2579       +4     
==========================================
+ Hits         2448     2450       +2     
- Misses        127      129       +2     

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