feat(chart): allow priorityClassName configuration

[apt-itude]

This is essential configuration to ensure that Kargo components are given appropriate scheduling priority according to the standards you've established for your cluster. Without being able to configure the priority class, Kargo pods may frequently be evicted in order to schedule pods that do have a priority class configured.

This my first PR to this repository, so please provide any contribution guidance that I need to follow!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.55%. Comparing base (de6d9c0) to head (4ce5ebb).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5538   +/-   ##
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  Coverage   55.55%   55.55%           
=======================================
  Files         431      431           
  Lines       31952    31952           
=======================================
  Hits        17752    17752           
  Misses      13190    13190           
  Partials     1010     1010           

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