[eas-build-job] Add custom action schema and utils

[sswrk]

Why

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How

How did you build this feature or fix this bug and why?

Test Plan

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[linear-code]

ENG-22387

[sswrk]

• [eas-cli] Validate custom actions in workflow files #3931
• [build-tools] Load custom action catalog at build runtime #3930
• [steps] Expand custom actions at parse time #3929
• [eas-build-job] Add custom action schema and utils #3928 👈 (View in Graphite)
• main

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[codecov]

Codecov Report

❌ Patch coverage is 89.53488% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.30%. Comparing base (72ca5c2) to head (ff953ae).

Files with missing lines	Patch %	Lines
packages/eas-build-job/src/action.ts	89.42%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3928      +/-   ##
==========================================
+ Coverage   59.24%   59.30%   +0.07%     
==========================================
  Files         935      936       +1     
  Lines       41061    41147      +86     
  Branches     8650     8667      +17     
==========================================
+ Hits        24323    24400      +77     
- Misses      16644    16653       +9     
  Partials       94       94              

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[github-actions]

⏩ The changelog entry check has been skipped since the "no changelog" label is present.