Fix Custom Resource pagination hang

[jacobtomlinson]

Follow on to #523 to close #544.

In #544 we are observing a hang when listing custom resources. It appears that built in resources omit the continue metadata parameter when reaching the end of the list, so we were testing for that to decide when to finish paging. However for custom resoureces it still includes the continue paratmeter but just returns an empty string. This was resulting in us paging forever.

This PR extends the check to ensure the continue parameter contains a value. I've also added come fixtures to make it easier to create and test custom resources in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.02%. Comparing base (87063fc) to head (5b41676).
Report is 156 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
+ Coverage   94.61%   95.02%   +0.40%     
==========================================
  Files          29       30       +1     
  Lines        3141     4084     +943     
==========================================
+ Hits         2972     3881     +909     
- Misses        169      203      +34     

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