Fix/gateway validate auto fields

[harshadixit12]

Summary

In deck gateway validate, we are not filling in plugin defaults and auto fields - which drifts apart from the implementation in deck gateway apply/sync - and leads to differences in behaviour.

In this PR, when plugins are present, I'm filling in the auto fields and defaults similar to how we do for gateway sync/apply.

Fixes

#1858

Performance impact

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.70%. Comparing base (94ccecd) to head (e8aeef1).

Files with missing lines	Patch %	Lines
cmd/gateway_validate.go	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2278      +/-   ##
==========================================
- Coverage   36.79%   36.70%   -0.09%     
==========================================
  Files          82       82              
  Lines        7922     7941      +19     
==========================================
  Hits         2915     2915              
- Misses       4740     4759      +19     
  Partials      267      267              

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