check for service account existing in validation

[drmorr0]

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Description

Add a validation rule to check if the service account for pods exists

Testing done

• manual testing
• new tests for the service_account_exists rule

[codecov]

Codecov Report

Attention: Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.

Project coverage is 72.91%. Comparing base (e27ffc0) to head (9526c64).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...li/src/validation/rules/service_account_missing.rs	77.08%	11 Missing ⚠️
sk-cli/src/validation/validation_store.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   72.87%   72.91%   +0.04%     
==========================================
  Files          55       56       +1     
  Lines        2971     3020      +49     
==========================================
+ Hits         2165     2202      +37     
- Misses        806      818      +12     

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