fix(webhook): Handle nil in mutation webhooks

[irvinlim]

Fixes #59.

• Extract JobPatcher and JobConfigPatcher from Mutator. There are many hidden assumptions in the MutateX methods, as we have seen that not only does it make the tests brittle, it also does not fully catch edge cases that may fall through the cracks.
• The Patcher types will simply just compose multiple MutateX methods in the desired order, and we will write tests based on the Operation and objects to be patched.
• Other fixes:
  • PodTemplateSpec should only add RestartPolicy on Job creates/updates, not JobConfig.
  • Handle nil for JobTemplate to add MaxAttempts.

[codecov]

Codecov Report

Merging #62 (984881e) into main (3e93e53) will decrease coverage by 0.09%.
The diff coverage is 39.13%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   61.15%   61.06%   -0.10%     
==========================================
  Files         166      168       +2     
  Lines        8406     8447      +41     
==========================================
+ Hits         5141     5158      +17     
- Misses       3024     3047      +23     
- Partials      241      242       +1     

Impacted Files	Coverage Δ
pkg/execution/mutation/patcher_jobconfigs.go	0.00% <0.00%> (ø)
...ution/webhooks/jobconfigmutatingwebhook/webhook.go	0.00% <0.00%> (ø)
...g/execution/webhooks/jobmutatingwebhook/webhook.go	0.00% <0.00%> (ø)
pkg/execution/mutation/patcher_jobs.go	65.00% <65.00%> (ø)
pkg/execution/mutation/mutation.go	88.02% <87.50%> (-1.17%)	⬇️

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