[eas-build-job] Add hooks schema

[sjchmiela]

Summary

• introduce a shared Hooks type/schema with reusable Zod/Joi validation so any string key maps to step arrays
• wire the new hooks definition through the generic, Android, and iOS job schemas and exports
• expand existing tests to cover accepting arbitrary hook names, empty hook arrays, and invalid hook step payloads

Not sure if it makes sense to make this "smart". I think let's just roll this out manually? Like, have eas/install_node_modules steps be sort of manually wrapped with hooks? Or maybe we'll sprinkle them when parsing the steps. I'll see. What I think is right is not to limit the keys to only a few known values.

Test plan

CI should pass.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.55%. Comparing base (50b9be9) to head (2d7e199).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3408      +/-   ##
==========================================
+ Coverage   52.54%   52.55%   +0.01%     
==========================================
  Files         806      806              
  Lines       33610    33616       +6     
  Branches     6985     6986       +1     
==========================================
+ Hits        17657    17663       +6     
  Misses      15871    15871              
  Partials       82       82              

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